It was late on a Thursday afternoon in the marbled halls of the Senate, and a small group of negotiators — one Republican, one Democrat and one independent — had just about finished a painstakingly put together border security compromise it took them months to forge.

But what should have been a triumphant moment felt more like an ordeal for the lone Republican in the trio.

“I feel like the guy standing in the middle of the field in a thunderstorm, holding up the metal stick,” Senator James Lankford, the Oklahoma Republican who was his party’s lead broker of the deal, told reporters last week.

The plight of Mr. Lankford, a slim, understated Baptist minister with a neatly combed shock of red hair and a baritone voice that regularly delivers deadpan quips, reflects the extraordinary rise and fall of the border and Ukraine deal that is expected to collapse in a test vote in the Senate on Wednesday — and the political forces within the Republican Party that brought it down.

For months, Mr. Lankford, a staunch conservative, labored over the package alongside Senators Christopher S. Murphy, Democrat of Connecticut, and Kyrsten Sinema, an Arizona independent, demanding strict immigration policies his party insisted must be a part of any bill to send a fresh infusion of aid to Ukraine. But when Mr. Lankford managed to extract them, he found his fellow Republicans unwilling to embrace the plan, in a vivid illustration of how the political ground for any compromise on immigration has vanished for a party that has decided the issue is too valuable as a political weapon to resolve.

Mr. Lankford, who previously ran the largest Christian youth camp in the country and has spoken often about how his faith guides his policy positions, has been left to pick up the pieces, a process he dryly likened on Tuesday to having been run over by a bus — and then having it back up over him again.

The soft-spoken second-term Republican, who generally refrains from seeking the political limelight, did not volunteer to helm the border negotiations when they began in the fall. Senator Mitch McConnell, Republican of Kentucky and the minority leader, deputized him as the top Republican on the Senate’s border security subcommittee. Or as Mr. Lankford put it, he drew the “short straw when it came time to be able to negotiate all this.”

His Republican colleagues warned him to be careful.

“I told him he was going to be like a goalie on a dart team weeks ago,” said Senator John Cornyn, Republican of Texas, who was supportive of the talks but came out this week in opposition to the bill. “He knew this was a treacherous path, but I have nothing but praise for Senator Lankford. I think he’s done the best job you could possibly do under the circumstances. It’s just a very, very, very difficult, complex situation.”

Just as Mr. Lankford and his fellow negotiators neared a deal, former President Donald J. Trump stepped in, trashing the bill both before and after it was released on Sunday and opening the floodgates of Republican resistance. That left Mr. Lankford fighting to keep the deal alive while being attacked by members of his own party, including in his home state, where the Republican Party tried to censure him late last month for “playing fast and loose with Democrats on our border policy.” (The resolution was later rescinded.)

Mr. Lankford said he was only the latest in a long line of lawmakers who had been burned by failed efforts to push through a bipartisan immigration deal.

“This only happens about every decade or so — to try to work on border security — because it’s so contentious, and it takes people a decade to forget what happened to those persons,” he said on Monday, “and in the process, to be able to take the risk again.”

The bill he helped write would impose the most significant border restrictions Congress has considered in decades, including measures to raise the bar for asylum claims, expand detention capacity and shut down the border if more than an average of 5,000 migrants per day attempt to cross in a week, or more than 8,500 try to cross in any given day. It reflects a paradigm shift in how Congress has considered modern immigration and border policy, with no mention of pathways toward legalization for undocumented immigrants currently in the country, previously a plank of any immigration policy Democrats might negotiate.

Still, Republicans have rejected the plan, with the hard right calling it too weak and more mainstream members including Mr. McConnell saying they are merely bowing to the political reality that it has no path through Congress.

The irony is not lost on Mr. Lankford.

Republicans “actually locked arms together and said: ‘We’re not going to give you money for this. We want a change in law,’” Mr. Lankford said on “Fox News Sunday” late last month. “Now it’s interesting, a few months later — when we’re finally going to the end — they’re like: ‘Oh, just kidding. I actually don’t want a change in law, because it’s a presidential election year.’”

Democrats and Republicans both regard Mr. Lankford as serious, policy-focused, trustworthy and firmly conservative.

“I decided to do this because James was going to be my partner,” said Mr. Murphy, who spent countless hours in a room in the Capitol with Mr. Lankford, poring over the details of the package. “He comes to politics through a desire to enact change, not because he loves the rough-and-tumble of political life. At some level, he’s a bit of a throwback. That probably causes him some problems when you’re thrown into the mix of one of the most difficult, most controversial issues in American politics.”

Mr. Lankford is “a highly respected and conservative member of our conference who is willing to do the hard thing,” Senator John Thune of South Dakota, the No. 2 Republican, said.

His attention to detail was part of the reason it took so long to cement a deal, Mr. Murphy said.

“James was committed to knowing every single piece of this bill inside and out, so that ultimately he could defend it,” he said. “Obviously, Trump has made his life difficult.”

On Monday, as Speaker Mike Johnson, Republican of Louisiana, declared the bill “even worse than we expected,” Mr. Trump singled out Mr. Lankford for disdain, telling the conservative commentator Dan Bongino on his show that the agreement was a “very bad bill” for Mr. Lankford’s career — “and especially in Oklahoma.”

Mr. Lankford, who won re-election in 2022 with 64 percent of the vote, is not due to face voters again until 2028, and it was not clear whether his involvement in the deal would hurt him in a state where he enjoys a reservoir of good will.

Mr. Lankford “had honorable intentions, but he took on a herculean task,” Chad Alexander, the former chairman of the Oklahoma Republican Party, said in an interview. “The sentiment has gotten even more intense since he began this four months ago. And now it’s a powder keg.”

In some ways, Mr. Lankford’s involvement in the effort recalls a very different political moment decades ago, when evangelical Christians were influential voices on the right in favor of humane immigration policies, including welcoming refugees and a pathway to citizenship for undocumented people.

“He is absolutely guided by his faith and the value he places on each life,” Pat McFerron, an Oklahoma pollster and political consultant, said in a text message. “Every person is made in God’s image. The passion he has here is no different than his passion for helping the unborn.”

Mr. Lankford has previously found himself swept up in the crosscurrents of Republican politics. After the Jan. 6, 2021, attack on the Capitol by a pro-Trump mob, Mr. Lankford apologized to Black constituents in Oklahoma for having supported Mr. Trump’s attempt to overturn the election in Congress, saying he had not realized that his stance would be seen as an attack on their voting rights.

He has also taken on other intractable political matters before. He worked closely with Senator Tim Scott, the lone Black senator in the G.O.P., on Republican legislation to address systemic racism in law enforcement, and he sought to bridge racial divides and attitudes about race within the G.O.P. during widespread racial justice protests in 2020.

None of it has been trickier than the border effort. On Monday, as the bill’s chances looked increasingly dismal, he went on a media blitz, trying to explain a provision in the package that Republicans have misrepresented as allowing 5,000 undocumented immigrants into the country every day. He pointed out that Republicans have repeatedly said there is a crisis at the border and that Congress needed to pass new legislation — a message Democrats are already deploying against the G.O.P.

“Are we as Republicans going to have press conferences and complain the border’s bad, and then intentionally leave it open after the worst month in American history in December?” Mr. Lankford said on “Fox & Friends,” adding that few had thought Republicans could secure such significant policy concessions from Democrats.

“No one would have believed it,” Mr. Lankford said. “And now no one actually wants to be able to fix it.”

On Tuesday, even as his colleagues were calling the compromise dead, Mr. Lankford refused to give up.

“The Scripture references say you work as long as there’s daylight, so I’m going to keep working until we know it’s got no chance to be able to move anymore, because I really think the issue needs to be resolved,” he said.

That was not the prevailing attitude among Republicans, particularly hard-right lawmakers who were exultant at the collapse of his effort.

To a long thread Mr. Lankford posted on social media trying to defend the bill, Representative Matt Gaetz, Republican of Florida, responded bluntly: “Just take the L.”

Karoun Demirjian and Catie Edmondson contributed reporting.