Hebron, Kentucky - President Donald Trump tightened his grip on the Republican Party on Tuesday as Kentucky voters ousted one of the few GOP lawmakers willing to challenge him openly.

Congressman Thomas Massie's defeat, called by news networks including NBC and CNN about two hours after polls closed, marked another victory in Trump's campaign to punish Republican dissent.

It follows the rout of Indiana state lawmakers who defied the president on redistricting and the weekend collapse of Louisiana Senator Bill Cassidy's reelection bid.

Massie's loss came after months of attacks from Trump, more than $32 million in ad spending, and an unusual campaign appearance by Defense Secretary Pete Hegseth on behalf of Trump-backed challenger Ed Gallrein, turning a once-sleepy Kentucky primary into a national loyalty test.

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Several states held primaries Tuesday to pick candidates for the November midterm elections, but the standout contest was in Kentucky.

Massie, a seven-term congressman, has been one of the president's most persistent internal critics.

The race was being watched as a measure of whether Trump's grip on Republican voters remains strong despite war, inflation, and sliding national approval ratings – and whether there is still room in the party for lawmakers willing to break with him.

Massie has angered Trump by opposing US military action in Iran and Venezuela, criticizing aid to Israel, resisting parts of the president's agenda, and helping push for the release of files related to multimillionaire sex offender Jeffrey Epstein.

Trump endorsed Gallrein, a farmer and retired Navy SEAL, in what US media have described as the most expensive House primary in American history – much of the spending from pro-Israeli groups opposed to Massie.

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The president has spent months attacking the avowedly libertarian 55-year-old former engineer and inventor, labeling him a disloyal "moron," a "nut job," and a "major sleazebag."

"Dealing with him is just horrible. I don't think he's a Republican...He's not a libertarian," Trump told reporters after polls opened.

"Sometimes they say he's really a Dumb-ocrat. He votes against us all the time," added the president, who has recently taken to insulting Democrats with his alternative pronunciation.

In the northern Kentucky city of Covington, Rob Barkley – a disillusioned former Trump supporter – said the president's attacks had only made him more supportive of Massie.

"He's on the Republican side so he has a conservative mindset," Barkley told CNN after casting his ballot. "But he's not as far-right leaning as Trump's politics."

Massie – who has voted with Trump around 90% of the time so far in the president's second term – cast the race as a test of independence inside the Republican Party.

"I'm not running against President Trump. Most of the people voting for me support President Trump like I do," Massie told Fox News.

Hegseth made his unusual appearance in Massie's district on Monday to campaign for Gallrein.

It is against the law for federal employees to engage in partisan political activity while on duty, but Hegseth's office said he was acting in a personal capacity and that no taxpayer funds would be used.

Trump later revealed that the Pentagon chief's time out from his official duties came just hours before the US was expecting to launch a new military assault on Iran, although hostilities were ultimately postponed.

Elsewhere Tuesday, Georgia voters are choosing candidates in Senate and gubernatorial primaries, but the state's supreme court races may provide the clearest bellwether in the key swing state.

Democrats are trying to unseat two incumbent justices in contests that have drawn endorsements from Barack Obama and other party heavyweights. No incumbent Georgia supreme court justice has lost reelection in more than a century.