HEBRON, Ky. — Rep. Thomas Massie, a constant thorn in the side of GOP leaders during his eight terms in Congress, is no stranger to a primary challenge. But this year, President Donald Trump’s fury with the Kentucky libertarian has reached a boiling point.

On Tuesday, voters in Kentucky’s 4th Congressional District will decide whether Massie becomes the latest casualty of Trump’s campaign to unseat foes inside the Republican Party as he faces off against Ed Gallrein, a former Navy SEAL and fifth-generation farmer who was personally recruited by the president.

“People aren’t going to be voting for me or voting for him on election day,” Massie told NBC News in an interview in his Capitol Hill office. “They’re either going to be voting for me or voting against me.”

The race has been the most expensive House primary in history in terms of ad spending, according to AdImpact, totaling more than $32 million — largely dominated by Trump-aligned groups and pro-Israel organizations pouring millions into negative ads against Massie.

On Saturday, Trump successfully ousted Sen. Bill Cassidy, R-La., after recruiting and backing a primary opponent, GOP Rep. Julia Letlow. And earlier this month in Indiana, Trump helped unseat GOP state lawmakers whom he blamed for foiling his redistricting there.

In Kentucky, Trump searched for months for a challenger to Massie, eventually landing on Gallrein, who had sought public office just once before, losing a state Senate race in 2024. When Gallrein entered the primary against Massie in October, blessed by a Trump post on Truth Social encouraging him to run, he was, by most accounts, a blank slate.

“I want to just — give me somebody with a warm body to beat Massie,” Trump said at a March event in the heart of Massie’s district. “And I got somebody with a warm body, but a big, beautiful brain and a great patriot.”

Gallrein has leaned into his identity as Trump’s chosen candidate, telling Kentuckians at a campaign stop on Thursday that he is “100% behind the president and what he is doing to turn our nation around.”

He declined to debate Massie and skipped county events where both candidates were invited to speak. He did not respond to a request for an interview from NBC News.

Trump and Massie’s contentious relationship dates back to Trump’s first term, when Massie blocked swift passage of a Covid relief bill during the pandemic. Trump then demanded that Massie be thrown out of the Republican Party, calling the congressman a “third rate Grandstander.”

That didn’t come to pass in 2020. But the tension grew worse through Trump’s second term.

Massie was one of only two House Republicans to vote against Trump’s “big, beautiful bill,” his signature tax and spending package, citing concerns about the national debt.

“I think Elon Musk gave up on this place,” joked Massie, an MIT-trained engineer who has worn a homemade debt-clock pin on his lapel for years. “He found it was easier to land rockets backwards than it was to get this place to cut even a dime of spending.”

But the final straw for Trump, according to Massie, was the congressman’s participation in a bipartisan push to force the Justice Department to release all files pertaining to the late convicted sex offender Jeffrey Epstein. Trump initially opposed the bill before eventually signing it into law.

“I vote with the party 90% of the time, but there’s 10% of the time where I think my constituents are better served by a different vote,” Massie said. “Releasing the Epstein files put me on the wrong side of the president for quite a while, but on the right side of my constituents, who had been promised that we would release the Epstein files.”

Massie, who has long opposed foreign aid and military involvement overseas, also sponsored a war powers resolution to stop Trump’s strikes on Iran and is one of the most vocal Republican critics of the war.

Trump has called Massie a “pathetic LOSER,” a “lightweight” and a “sick Wacko.”

“Thomas Massie is a disaster for our party. He comes from a state that I won by a landslide. We got to get rid of this loser. This guy is bad. He’s disloyal to the Republican Party,” Trump told the crowd when he came to Kentucky in March, drawing some boos at the mere mention of Massie’s name.

Yet some in attendance told NBC News that although they support Trump, they would vote again for Massie.

With Trump’s stranglehold on the GOP, Massie has only a handful of Republican allies left on Capitol Hill, among them Sen. Rand Paul of Kentucky and Reps. Warren Davidson of Ohio, Victoria Spartz of Indiana and Lauren Boebert of Colorado, the latter of whom faced backlash from Trump and his supporters after members of that group campaigned with Massie in Kentucky over the weekend.

“I support both of these men,” Boebert wrote on X. “And if that makes you angry, bless your heart.”

Spartz suggested the two are more alike than they appear. “They both should succeed. They are very similar,” she told NBC News. “They should be friends. This is just politics and elections.”

Davidson expressed similar hopes. “I support Thomas. I like President Trump. I like Thomas. I hope they make up,” he said, noting that Vice President JD Vance had once been a fierce Trump critic before reconciling. “JD, at one time, didn’t like President Trump. They’re super tight now.”

The race has also put House Speaker Mike Johnson in an awkward spot. Typically, GOP leadership backs its incumbents, but Trump has made ousting Massie a top priority, and he is hardly a reliable vote for Johnson’s fragile, narrow majority.

“It would be helpful to have a more reliable vote for our agenda and for the Republican Party. That’s certainly true,” Johnson told NBC News in the Capitol last week. “But I’ve effectively stayed out of the race, so I intend to do so.”

Rep. Andy Barr, who is running for Senate and has endorsed Gallrein, said Massie’s independent streak that once won him antiestablishment voters is now working against him.

“Thomas has a pretty loyal libertarian group in northern Kentucky, and they’re pretty immovable,” Barr said. “But what you see, which is different than in prior cycles, is the antiestablishment, rural MAGA vote cutting against Thomas. They’ve grown extremely weary of what they see as disloyalty to the president.”

Massie argues the outcome will come down to turnout, and he points to a sharp generational divide in the sparse polling of the district. Younger voters, he says, are breaking heavily for him. It’s older voters — “the Fox News demographic” — who are drifting toward Gallrein, according to Massie.

“The younger people do want America first. They do want fewer wars, because they might be the ones who have to go fight them,” Massie said. “This will be a referendum on whether the future of the party is here on May 19.”

As for his relationship with Trump, Massie was characteristically unbothered.

“There are a lot of congressmen here, he wouldn’t know their first name or their last name or what state they’re from,” Massie said. “He knows who I am. He knows what I stand for. And we’ve worked together on things in the past, and we’ll work together on things after this election.”

“I think we’re going to be fine on May 20,” he added.