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 has described as a "major sleazebag" and "the worst Republican congressman in history".

Massie, who has been in office since 2012, broke with Trump by voting against his "big, beautiful" tax and spending legislation 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.

On Monday, Trump criticised Massie in a series of social media posts, calling him "an obstructionist and a fool".

US Secretary of Defence Pete Hegseth campaigned for Gallrein in Kentucky, where he accused Massie of "constant obstruction". In response, Massie said that he saw Hegseth's presence in his district as a good sign.

"You don't send the Secretary of War to Kentucky during a war if you think your candidate is up 10 points. That's what you do when you realise your whole campaign is imploding," he told CBS News.

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Juan Orlando Hernández was controversially pardoned by Trump in December while serving a 45-year prison sentence in the US for drug trafficking.

The president, who claims the building is a ‘gift’ to the US, says it will include a hospital and research facilities.

Thomas Massie, the Republican congressman from Kentucky who led the release of the Epstein files, is running against a Trump-backed primary challenger.

Juan Orlando Hernández was serving a 45-year prison sentence for drug trafficking when he was pardoned by Trump.

The relationship endures despite a power imbalance, with both sides recognising that it is too important to fail.