Leonard Zeskind, a dogged tracker of right-wing hate groups, who foresaw before almost anyone else that anti-immigrant ideologies would move to the mainstream of American politics, died on April 15 at his home in Kansas City, Mo. He was 75.

The cause was complications of pancreatic cancer, Carol Smith, his wife, said.

Long before Donald J. Trump’s nativist rhetoric in 2023 accusing immigrants of “poisoning the blood” of the United States, Mr. Zeskind, a single-minded researcher, spent decades studying white nationalism, documenting how its leading voices had shifted their vitriol from Black Americans to nonwhite immigrants.

His 2009 book, “Blood and Politics: The History of the White Nationalist Movement From the Margins to the Mainstream,” resulted from years of following contemporary Klansmen, neo-Nazis, militia members and other right-wing groups. His investigations earned him a MacArthur “genius grant” in 1998.

“For a nice Jewish boy, I’ve gone to more Klan rallies, neo-Nazi events and Posse Comitatus things than anybody should ever have to,” Mr. Zeskind said in 2018.

Recently, “Blood and Politics” was one of 381 books removed from the U.S. Naval Academy library in a purge of titles about racism and diversity ordered by the Trump administration.

One of Mr. Zeskind’s central themes was that before the 1960s, white supremacists fought to maintain the status quo of segregation, especially in the South. But after the era of civil rights victories, he maintained, white nationalists began to see themselves as an oppressed group, victims who needed to mount an insurgency against the establishment.

Their principal adversaries were immigrants from the developing world who were tilting the demographics of the United States away from earlier waves of Northern Europeans.

Despite the subtitle of Mr. Zeskind’s book, asserting that white nationalists had moved “from the margins to the mainstream,” many reviewers in 2009 were skeptical, treating his work as a backward look at a fringe movement led by racist crackpots whose day was over. The United States had just elected its first Black president, and extremist movements such as Christian Identity, which preached that white Christians were entitled to dominate government and society, seemed antiquated.

The Los Angeles Times waved away those hate groups as questing after “an impossible future.” NPR noted that “while a handful of bigots” were still grumbling about the South’s defeat in the Civil War and spreading conspiracies about Jews, “some 70 million others have, in a testament to the overwhelming tolerance of contemporary American society, gone ahead and elected Barack Obama president.”

Mr. Zeskind insisted that white nationalists should not be underestimated, and he was especially concerned about their influence on Republican politics.

He identified those influences in the candidacies of David Duke, a former Klan leader who won a majority of white voters when he ran for statewide office in Louisiana in 1990, and in Pat Buchanan, who fared well in G.O.P. presidential primaries in the 1990s, running on a platform of reducing immigration, opposing multiculturalism and stoking the culture wars.

Mr. Buchanan’s issues offered a template for Mr. Trump, who leveraged similar ideas to wrest control of the Republican Party from its establishment.

Mr. Zeskind spoke about Mr. Trump in a 2018 town hall speech in Washington on the one-year anniversary of the march in Charlottesville, Va., by young white supremacists, whose zealotry the president had minimized. Mr. Zeskind said that Mr. Trump hadn’t created an upsurge in hatred of nonwhite people — he was a product of it.

“White supremacy and white privilege have been dominant elements of our society from the beginning,” he said. “It breeds a whole set of behaviors in people, and it should be deeply and widely discussed in every level of our society.”

Leonard Harold Zeskind was born on Nov. 14, 1949, in Baltimore, one of three sons of Stanley and Shirley (Berman) Zeskind. His parents, who ran a pension management business, moved the family to Miami when Leonard was 10. He graduated from Miami Senior High School, and then studied philosophy at the University of Florida and the University of Kansas, though he did not graduate.

Ms. Smith, his wife, said he was expelled from college in Kansas after taking part in a 1960s campus protest of the Reserve Officers Training Corps.

Mr. Zeskind earned a welding certificate from the Manual Career and Technical Center in Kansas City, and for 13 years worked as a welder and ironworker and on assembly lines. He was also a community organizer on Kansas City’s East Side, seeking to lower tensions between white working-class families and their Black neighbors.

He met Ms. Smith in 1979. She had grown up on a dairy farm in Kansas, and through her he became aware that during the farm crisis of the 1980s, a conspiracy movement known as Posse Comitatus had spread among economically ravaged farmers, who were convinced that they had been targeted by Jewish bankers and others because they were white Christians.

Mr. Zeskind was invited to speak about Posse Comitatus to a group of progressive farmers in Des Moines, and he mobilized Jewish groups nationally to counter the conspiracy movement.

From 1985 to 1994, he was the research director at the Center for Democratic Renewal (previously the National Anti-Klan Network). In 1983, he founded the Institute for Research and Education on Human Rights, a study and watchdog group focused on hate groups.

Besides Ms. Smith, he is survived by a brother, Philip. His first marriage, to Elaine Cantrell, ended in divorce.

At the 2018 town hall meeting in Washington, Mr. Zeskind called on Democrats in Congress to vehemently oppose a little-noticed bill sponsored by Representative Steve King, Republican of Iowa, to end birthright citizenship, the post-Civil War guarantee that anyone born in the United States is a citizen. The cause had become a focus of anti-immigrant groups warning of threats to the “white race.”

“They want to smash up the 14th Amendment,” Mr. Zeskind said, addressing Democratic officials, “and I think you guys should scream about it.”

The following year, in an article in The New York Times about how Mr. King, a backbencher in his party, had anticipated many of Mr. Trump’s anti-immigrant stances, the congressman said in an interview, “White nationalist, white supremacist, Western civilization — how did that language become offensive?”

Republican leaders in the House stripped Mr. King of his committee assignments over the remark, and he lost re-election in 2020.

But the issue did not die. One of President Trump’s first moves in January was an executive order to end birthright citizenship.

Last week, the Supreme Court agreed to hear arguments over Mr. Trump’s order.

A deadly blast on Sunday near a UNESCO world heritage site in Yemen’s capital was caused by a Houthi missile, not a U.S. airstrike, a spokesman for U.S. Central Command said on Thursday.

The health ministry of the Houthi-led government said earlier this week that an American airstrike had hit a densely populated neighborhood of Sana, the Yemeni capital, killing 12 people and injuring 30 others. The blast struck an area adjacent to Sana’s Old City, a UNESCO world heritage site filled with ancient towers.

Dave Eastburn, a spokesman for U.S. Central Command, which oversees operations in the Middle East, said in a statement that while the damage and casualties described by local health officials most “likely did occur,” they were not the result of an American attack. While the United States had conducted military operations over Sana that night, the closest American strike was more than three miles away, he added.

The Pentagon’s assessment that the damage was caused by a “Houthi Air Defense missile” was based in part on a review of “local reporting, including videos documenting Arabic writing on the missile’s fragments at the market,” Mr. Eastburn said. The Pentagon did not provide those videos or evidence of its claims in its statements.

An initial review by The New York Times of local reporting and open-source material in Yemen found a video showing a missile fragment with Arabic writing posted to social media, however it was from a different location from the market in Sana’s Old City.

Mohammed al-Bukhaiti, a member of the Houthis’ Politburo, said in a phone interview that the American denial was an attempt to smear the Houthis. He reiterated that the group believed that the United States targeted the neighborhood on Sunday, “just as it previously targeted ports, cemeteries and citizens’ homes, resulting in the deaths of hundreds.”

The Trump administration has in recent weeks carried out an intense bombing campaign over areas of Yemen controlled by the Houthis, an Iran-backed militia that rules much of the country’s north with an iron fist. The militia has been firing rockets and drones at Israel and attacking ships in the nearby Red Sea, in a campaign that its leaders say is in solidarity with Palestinians in Gaza.

Asked by The Times earlier this week about the Sunday strike, the U.S. Defense Department did not comment on the Houthis’ claims. Instead, it said in a statement that the United States was targeting “Iran-backed Houthi locations every day and night in Yemen” with the intention of restoring freedom of navigation and deterring the Houthis from further attacks.

The details of strikes have been challenging to verify for journalists on the ground. Houthi officials have stymied journalists and citizens from documenting airstrikes — including the site of the blast on Sunday — warning that such information could be exploited by foreign enemies. Asked about those restrictions, Mr. al-Bukhaiti said that “it is common for the targeted area to be cordoned off to facilitate rescue operations and to prevent civilians from gathering, in case of renewed strikes or the targeting of medics.”

For nearly a decade, Yemen has been at war. After the Houthis, a once-scrappy tribal militia, took over the Yemeni capital, the country was pummeled by a Saudi-led military coalition supplied with American bombs in an effort to defeat them.

That coalition expected swift victory. Instead, hundreds of thousands of people died from fighting, hunger and disease. And since the coalition pulled back several years ago, partly because of international pressure, the Houthis have deepened their grip on power, evolving into a de facto government in northern Yemen.

The Houthis began their latest attacks in late 2023, after Hamas stormed into southern Israel, killing about 1,200 people and taking hundreds more captive into Gaza. Israel responded by bombarding the territory, killing more than 50,000 people, according to the Gazan health authorities, whose figures do not distinguish between combatants and civilians.

The Houthis have described their attacks on ships as an attempt to pressure Israel and outside nations to increase the free flow of humanitarian aid to Gaza, where more than two million Palestinians have struggled to obtain food and water.

Because Houthi territory abuts a vital waterway that ships must pass to reach the Suez Canal, the attacks have disrupted global trade, pushing container ships to take a longer route around the southern tip of Africa. The Houthis say they are attacking ships with Israeli or American ties, although many of the targeted vessels have had no clear link to either country.

The United States and Britain began bombarding Houthi targets last year, saying they were attempting to halt the attacks on shipping and Israel.

The militia briefly stopped firing rockets at Israel during a two-month cease-fire between Israel and Hamas this year. But after a truce ended in mid-March, Israel renewed its offensive in Gaza and the Houthis resumed firing ballistic missiles at Israeli territory.

The Trump administration began its own campaign of airstrikes in March.

American strikes that hit a vital port in the region of Hudaydah this month killed at least 74 people, health officials under the Houthi-led government said.

U.S. Central Command said that it had targeted the port because shipments of fuel were still flowing into it in defiance of American sanctions, allowing funds to flow into the Houthis’ coffers. It did not provide its own assessment of how many people had been killed in the bombardment.

Secretary General António Guterres of the United Nations expressed “grave concern” over those strikes, saying in a statement that at least five humanitarian workers were said to be among the injured and urging all parties in the conflict to respect international law and protect civilian infrastructure.

So far, the American campaign has not appeared to have deterred the Houthis, who have continued to announce attacks on Israel and ships. Yemeni scholars who study the group warn that American airstrikes will simply play into the militia’s agenda.

Arijeta Lajka and Aric Toler contributed reporting.

On April 7, the Supreme Court ruled that the government must give Venezuelan migrants notice “within a reasonable time” and the chance to legally challenge their removal before being deported to a maximum-security prison in El Salvador.

Exactly how much notice the Trump administration considered appropriate in response to the Supreme Court’s edict was revealed in a document unsealed during a hearing on Thursday in Federal District Court in Brownsville, Texas.

Before Saturday, when the Supreme Court issued a second order, which blocked the deportation of a group of Venezuelan migrants under the Alien Enemies Act of 1798, detainees slated for deportation were given a one-page form that stated “if you desire to make a phone call, you will be permitted to do so,” according to the unsealed document, a four-page declaration by an official from Immigration and Customs Enforcement.

They then had “no less than 12 hours” to “express an intent” to challenge their detention, and another 24 hours to file a habeas corpus petition asking for a hearing before a judge, the declaration said. The form itself is written in English, but “it is read and explained to each alien in a language that alien understands.”

The hearing was part of a case whose plaintiffs are three Venezuelan men being held at El Valle Detention Facility, roughly 50 miles from Brownsville.

Lawyers for detainees held elsewhere, who have sued in the Northern District of Texas, have disputed the government’s claims about being given notice. They also have said that the form was not explained to detainees and that they were simply told to sign the document, which the ICE declaration identified as Form AEA-21B.

The details about notice came during a two-hour hearing before Judge Fernando Rodriguez Jr., who unsealed the ICE declaration after rejecting the government’s stance that it should remain sealed because it contained sensitive law-enforcement details.

Judge Rodriguez also some expressed skepticism about President Trump’s assertion in an executive order that the men could be deported under the Alien Enemies Act, a wartime law, because of claims by the government that they are members of a gang. The government tried to defend Mr. Trump’s wording that activity by Tren de Aragua amounted to “an invasion” and “predatory incursion,” but it was unable to provide what the judge requested: documentation from the time the act was passed that supported that argument.

“You’re giving me your view of what the words mean,” he said. “What I’m looking for is what the words meant at the time.”

Immediately after the hearing, Lee Gelernt of the A.C.L.U., one of the lawyers for the three plaintiffs, said the notice given to his clients was insufficient.

“Twelve hours is obviously too short to find out who to contact, and 24 hours to file a habeas corpus petition is obviously inconsistent with any notion of due process, or the Supreme Court’s ruling,” he said.

Judge Rodriguez is one of at least five district court judges who have issued temporary restraining orders barring the administration from deporting individuals from their districts under the Alien Enemies Act. He and another of those five judges were appointed by Mr. Trump.

At the end of the hearing on Thursday, Judge Rodriguez extended his restraining order by one week, to May 2.

Alan Feuer contributed reporting.

President Trump made an unusually sharp appeal to President Vladimir V. Putin of Russia on Thursday, calling on him to stop his bombing campaign in Ukraine and agree to a peace deal after the deadliest attack on Kyiv in nearly a year.

“Not necessary, and very bad timing. Vladimir, STOP! 5000 soldiers a week are dying. Let’s get the Peace Deal DONE!” Mr. Trump wrote on social media.

Russia’s missile attack came a day after the Trump administration threatened to abandon peace talks if Ukraine did not accept a U.S. peace proposal that heavily favored Russia.

Mr. Trump’s comments were striking because he has mostly avoided even mild criticism of Mr. Putin in his handling of the talks so far. Instead, he has directed most of his anger toward President Volodymyr Zelensky of Ukraine, calling him a “dictator” and describing him as the main impediment to a peace deal.

While Mr. Trump made clear he was running out of patience for the two sides to agree on a peace deal, he also sought to pre-emptively divert blame should the negotiations fall apart, a sign that he is perhaps more pessimistic than he was when he regained the presidency brimming with confidence about his talent as a negotiator.

The Russia-Ukraine war, which Mr. Trump had previously said he could resolve in “24 hours,” was now, he suggested, a matter of great difficulty and complexity.

“This isn’t my war,” Mr. Trump said during a Thursday Oval Office meeting with Norway’s prime minister. “It’s Biden’s war.”

Mr. Trump refused to draw a moral distinction between Russia and Ukraine, or to blame Mr. Putin for his invasion — something he has repeatedly refused to do in his second term.

He reiterated, though, that he was “not happy” with Russia’s deadly attack on Kyiv overnight, which came as the Trump administration demanded that Mr. Zelensky accept a peace settlement that would grant Russia essentially all the territory it had gained in the war, while offering Ukraine tenuous security assurances from the Europeans.

The plan, which would also block Ukraine from ever joining NATO, was rejected by Mr. Zelensky, infuriating Mr. Trump.

Pressed about what concessions Russia had offered, Mr. Trump said that it had agreed to stop “taking the whole country.” Mr. Putin’s military has failed to make any significant territorial gains recently.

Mr. Trump added that it would be hard for Ukraine to gain back the territory it had lost during Russian invasions that took place during the presidencies of Barack Obama and Joseph R. Biden Jr. Ukraine getting back Crimea, which Russia annexed in 2014, would be “a very difficult thing to do,” he said. But Mr. Trump claimed that he was “using a lot of pressure” behind the scenes on both Russia and Ukraine.

Senior administration officials, including Secretary of State Marco Rubio, have signaled that Mr. Trump is getting impatient and might walk away from the negotiations if Russia and Ukraine do not come to an agreement soon. If the United States withdraws from the talks, and cuts off its supply of weapons to the Ukrainian military, Mr. Putin would have a greater chance of capturing more of the country.

When a reporter on Thursday asked Mr. Trump whether he would impose new sanctions on Russia after the overnight bombing of Kyiv, Mr. Trump declined to say, adding only that he should be asked again in a week. He said that he wanted to see what progress his team could make through negotiations.

Mark Rutte, NATO’s secretary general, also met with Mr. Trump on Thursday at the White House, to discuss the war in Ukraine and an upcoming NATO summit. Mr. Rutte said afterward that he had a “very good meeting” with Mr. Trump, and that he did not think the United States would walk away from the Russia-Ukraine talks.

While Mr. Trump has said he believes Mr. Putin wants to make peace, Mr. Rutte said on Thursday that he did “not know” whether that was the case.

“There is something on the table now, I think, where Ukrainians are really playing ball, and I think the ball is clearly in the Russian court,” Mr. Rutte added.

Mr. Trump on Thursday also raised expectations for another difficult diplomatic effort his administration is undertaking this weekend with a hostile foreign government, when an American negotiating team begins technical talks with the Iranians in Oman.

“I think we’re doing very well on that, an agreement with Iran,” Mr. Trump said.

The Israeli government has been pushing Mr. Trump to support a military campaign that would destroy Iran’s nuclear facilities. The president has so far withheld his support for Israel’s mission, preferring to give diplomacy a chance. But he has also insisted that he will never allow Iran to obtain a nuclear weapon.

A brewing weeklong fight inside the Democratic National Committee burst into the open on Thursday as the party’s chairman, Ken Martin, rebuked one of his vice chairs and moved to stop him from intervening in Democratic primary races while serving as a top party official.

The vice chair, David Hogg, 25, had announced last week that he planned to spend money in Democratic primaries through his outside group, Leaders We Deserve, and that he hoped to raise $20 million for the effort.

That set off a storm of criticism from Democrats angry at the idea that a top party official would be putting his finger on the scale in primary contests. On Thursday, Mr. Martin responded publicly for the first time, declaring, “No D.N.C. officer should ever attempt to influence the outcome of a primary.”

Mr. Martin said he had “great respect” for Mr. Hogg and understood his goals, yet he issued what amounted to an ultimatum: Mr. Hogg was “more than free” to fund primary challenges, just not as an officer of the D.N.C.

Mr. Martin made his comments on a call with reporters announcing plans to expand grants to the party’s operations in red states.

At a private meeting last month, all of the committee’s officers — except Mr. Hogg — signed a pledge promising to remain neutral in primary races.

In a lengthy statement on Thursday, Mr. Hogg reiterated why he was backing primary challengers, saying, “This moment requires us to have the strongest opposition party possible to stop Trump.”

Mr. Hogg has done a blitz in the news media, appearing on cable shows to make his case after The New York Times first reported his plans, which he stipulated would be limited to races for safe Democratic seats. Mr. Hogg said his goal was to elect a younger generation of Democrats and replace older incumbents he saw as less effective. Still, as he faced blowback on Capitol Hill, his group donated $100,000 to the Democratic Congressional Campaign Committee.

Jane Kleeb, the chair of the Nebraska Democratic Party and the president of the Association of State Democratic Committees, said Mr. Martin would introduce a series of previously planned party changes that would include putting neutrality in the bylaws — meaning Mr. Hogg could not serve in his position if he were still pursuing his plan.

The package will go before the party’s membership in August, she said.

Mr. Hogg said in his statement that Democrats were “trying to change the rules because I’m not currently breaking them.”

“As we’re seeing law firms, tech companies, and so many others bowing to Trump, we all must use whatever position of power we have to fight back,” he added. “And that’s exactly what I’m doing.”

Mr. Martin, for his part, argued in an opinion essay in Time magazine that neutrality was essential for the party leadership.

“Our role is to serve as stewards of a fair, open, and trusted process — not to tilt the scales,” he wrote, adding it was to guard against ”misuse or abuse of power by those in official positions.”

Ms. Kleeb said the importance of party neutrality was made clear during the divisive 2016 primary race between Hillary Clinton and Senator Bernie Sanders of Vermont, when party leaders supported Mrs. Clinton.

“David got elected to be a D.N.C. officer,” Ms. Kleeb said of Mr. Hogg’s vice-chair post. “He did not get elected to primary Democrats.”

Ms. Kleeb said she had spoken with Mr. Hogg privately and told him that he could remain a part of D.N.C. leadership if he walled himself off from his outside group’s endorsement decisions, as some union leaders have done.

”He can’t have both,” she said. “He has to make a decision.”