Congressman Thomas Massie, one of the most vocal Republican critics of Donald Trump, has lost his fight for re-election to a challenger endorsed by the president.
Ed Gallrein, a former Navy Seal, will now compete in November's midterm election after his victory in the Kentucky primary on Tuesday.
The contest, the most expensive primary in the history of the House of Representatives, had been widely seen as a key test of President Trump's decade-long grip on the Republican Party.
Trump repeatedly called for voters to back Gallrein over the incumbent Massie, who he described as a "major sleazebag" and "the worst Republican congressman in history".
Massie, who has been in office since 2012, publicly broke with Trump by voting against his "big, beautiful" tax and spending bill last year over concerns about the national debt. He has also voted to curtail Trump's attacks on suspected drug boats in the Caribbean and his ongoing war in Iran.
Massie also joined with Democrats - and a handful of Republicans - to force Trump's Department of Justice to release all of its files on Jeffrey Epstein.
He lost by 55 percent to 45 percent in the vote on Tuesday, marking a major victory for Trump who has successfully ousted a number of dissenting Republicans by endorsing a more loyal challenger.
At his victory party, Gallrein told supporters he had come "a long way" from his childhood at his family's dairy farm to become the nominee.
"The winner of the race tonight is not so much Ed Gallrein," he said. "It's the Republicans of Kentucky district four and their families who I will be a champion for in Washington."
He went on to thank Trump for his support and his "counsel" during the race.
Many of the president's biggest donors backed Gallrein, and more than $32m (£23m) was spent on campaign advertising in the Kentucky district.
In his concession speech, Massie said voters had indicated they wanted someone who would "go along to get along", but added: "There is a yearning in this country for somebody who will vote for principles over party."
He criticised the administration's foreign aid spending, the plans to build a White House ballroom, the large donations to his opponent and a decision by Secretary of Defence Pete Hegseth to campaign in Kentucky for Gallrein on Monday.
"They panicked and sent the Secretary of War here, and you stopped the war for a day," he said, as his supporters cheered his name and chanted "America first".
On Monday, Trump criticised Massie in a series of social media posts, calling him "an obstructionist and a fool".
Massie then touted his record of voting with Trump "90% of the time," but added that the president and his allies "want 100% compliance".
"It's only the 10% of the time they're mad about - when I won't vote for a war, when I won't vote for warrantless spying and when I won't vote to bankrupt the country," he said.
In a separate development in Kentucky on Tuesday, Congressman Andy Barr won the Republican nomination to replace retiring Senator Mitch McConnell who has been in the Senate for more than 40 years.
Trump had endorsed Barr, and his victory did not come as a surprise after Trump intervened to offer an ambassadorship to his main Republican challenger.
Trump has been aggressively wielding his influence with recent Republican endorsements, including by helping to tank the campaign of an incumbent senator who had voted to convict him during his 2021 impeachment trial.
In Texas, he has also given his endorsement to Ken Paxton rather than longtime Republican Senator John Cornyn.
"John Cornyn is a good man, and I worked well with him, but he was not supportive of me when times were tough," Trump said of his decision to back Paxton on Tuesday.
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