Just as senators were closing in on a deal that Republican negotiators said would constitute the most conservative border security bill in decades, Donald Trump was closing in on the G.O.P. presidential nomination.

And his vocal opposition to the compromise, which would also send tens of billions of dollars in aid to Ukraine, now threatens the chances of the entire package in a divided Congress.

Trump’s twin primary victories have forced Republicans to once again fall in line. Now, he is wielding a heavier hand over his party’s agenda in Congress than at any other time since leaving office.

Republicans are “in a quandary,” Mitch McConnell, the Senate minority leader, told his conference in a closed-door meeting on Wednesday, according to lawmakers who attended. What was supposed to be the sweetener for conservatives opposed to sending tens of billions of dollars to Ukraine had become as politically treacherous as the foreign aid itself, he said. Trump’s growing influence was dividing Republicans on an issue that once united the party.

A hard line on immigration

Trump burst onto the national political scene in 2015 with a dark warning about dangerous immigrants invading the country’s Southern border and with a slogan that outlasted them all, “Build the Wall.” More than eight years later, he is agitating against compromise: “We need a Strong, Powerful, and essentially ‘PERFECT’ Border and, unless we get that, we are better off not making a Deal,” he wrote on social media.

With more than half of Senate Republicans now officially backing Trump’s candidacy, those entreaties are becoming harder to ignore as mere prattle from Palm Beach.

Senators have been working on a border deal since just before Thanksgiving. But as the complicated talks have dragged on, Trump has begun collecting delegates and pressing for the party to coalesce around him and his agenda. His allies in Congress regard him as the party’s de facto leader and urge their colleagues to fall in line with his policies. Even Republicans who are not die-hard fans of Trump have said in recent days that they will do what they can to support him as the party’s nominee.

His policies include severe immigration measures that President Biden and Senate Democrats would never support. Trump and most House Republicans want to block migrants from living and working temporarily without visas in the United States while they await the outcome of their immigration claims. And conservatives seek a revival of the “Remain in Mexico” policy, which forced immigrants to wait elsewhere to see if they would be permitted entry. Speaker Mike Johnson, who talks about immigration regularly with Trump, has said that a Senate deal without those policies would be dead on arrival in the House.

The collapse of a deal

Yesterday, Republican supporters of a border deal were livid at the notion that Trump might tank their work. “We have to have people here who support Trump, who have endorsed President Trump, go to him and tell him what a compelling case this is,” Thom Tillis, a North Carolina senator, said.

Trump has always had far more power to derail things than to help lawmakers find consensus. The former House speaker Kevin McCarthy knew that — and worked hard last year to keep Trump quiet during negotiations with Biden to avoid a federal debt default. Trump didn’t weigh in until after the deal was done. McCarthy was able to pass the bill with a majority of G.O.P. support.

But back then, Trump was not yet winning nominating contests, and spending cuts were never the animating issue of his political identity. Attacking Biden’s border policy is already a main plank of Trump’s campaign. He has accused the president of opening the door for terrorists and for fentanyl to pour into the country. A Trump campaign ad claims that Biden’s immigration policies raise “the possibility of a Hamas attack” in the United States. And Trump vows to build “even more wall” along the Southern border.

On Thursday, McConnell tried to assure alarmed senators that he was still moving ahead with the border-Ukraine package. Still, the road is less certain than it was last year, before Trump began winning nominating contests.

Senator Tommy Tuberville, an Alabama Republican and a staunch Trump supporter, said the breathless atmosphere that enveloped Capitol Hill yesterday was nothing but hot air.

“We haven’t even seen the text yet and everyone is panicking,” he said. “Just get it done and see if you got enough votes.”

• Ingenuity, NASA’s tiny helicopter on Mars, ended its mission. It was searching for evidence that life was once on the planet.

• Someone struck a South Korean lawmaker on the head with a blunt object, leaving her with injuries that weren’t life-threatening. It was the second public attack on a Korean politician in less than a month.

• Obituary pirates are publishing dubious articles about everyday people’s deaths to capture Google traffic.

• Arkansas authorities recaptured one of two men who escaped from a jail there. The other remains missing.

Opinions

People are acutely pessimistic about humanity’s future right now. Periods of panic are a pattern throughout history, Tyler Austin Harper writes.

“Killers of the Flower Moon” asks Americans to confront the shame of their colonial history, Maggie Blackhawk writes.

Here are columns by Paul Krugman on “Bidencare” and by David Brooks on classic art.

36 Hours: Scuba dive through an underwater art gallery on Grenada.

Live fast, die young: Male antechinus, rodent-like animals from Australia, will forgo sleep for sex during their breeding season — after which they die.

Paris: Want to go to the summer 2024 Olympics? It’s going to be really expensive.

Lives Lived: Bill Hayes logged 2,141 episodes on “Days of Our Lives” over five and a half decades and recorded a best-selling single, “The Ballad of Davy Crockett,” in 1955. Hayes died at 98.

SPORTS

Women’s college basketball: No. 1 South Carolina beat No. 9 L.S.U., the defending national champions, 76-70 on the road.

N.F.L.: The Atlanta Falcons hired Raheem Morris — their interim head coach in 2020 — to the full-time position, ending a search originally focused on Bill Belichick.

Sports wagers: The former L.S.U. wide receiver Kayshon Boutte was arrested in Louisiana on an underage gambling charge.

Australian Open: Novak Djokovic lost today, ending a flawless record.

Lawsuit: A former employee of W.W.E. sued Vince McMahon, its longtime chairman and chief executive, accusing him of physical and emotional abuse, sexual assault and trafficking.

ARTS AND IDEAS

Artificial art: A movie concept artist and an artificial intelligence expert teamed up to test whether A.I. image generators were learning from, and reproducing, copyrighted images. Sure enough, their requests — and subsequent tests by Times reporters — led the systems to generate replicas of images from movies and video games. The A.I. companies say they are exempt from copyright restrictions, under a rule known as “fair use,” though many experts disagree. “What they’re doing is clear evidence of exploitation,” the artist, Reid Southen, said.