Donald Trump’s Republican party is on course for a damaging rejection at the ballot box in November, according to a maverick US congressman ousted by a challenger handpicked by the president.

Thomas Massie, a Republican from Kentucky, became the latest of Trump’s targets to be defeated in the party’s primaries this week. He had repeatedly broken with the president over military action against Iran, government spending and the release of the Jeffrey Epstein files.

As control of US Congress hangs in the balance, Massie warned on Sunday that Trump has “disenfranchised” many of the Americans who voted for him – and their party – in November 2024.

The libertarian-minded congressman, who will remain in the House of Representatives for the rest of year, promised to stay engaged in politics, even if “it’s from the outside”, and expressed no regrets for the actions that angered Trump, even though they helped prompt his defeat.

“It was completely worth it,” Massie told Meet the Press on NBC. “And I’ve got seven more months to keep going against the grain, which means voting for principles and for people over party.”

In a nod to “Trump derangement syndrome”, the label frequently used by the president to accuse his critics of an obsession with him, Massie said: “There’s a growing number of people on the right who have a form of TDS called Trump disappointment syndrome.

“And I think what’s going to happen to the party this fall, is they’ve disenfranchised a large portion of that constituency that Trump assembled to get us in the White House, in the Senate majority, and in the House majority.”

Massie pointed to several significant constituencies – including “Make America healthy again” campaigners, fiscal hawks pushing for sweeping government budget cuts, and voters who don’t want the US engaged in wars – who he claimed had been “alienated” by the administration’s actions. “And so, I’m worried that in November, this is going to cost the party a lot.”

Massie suggested in the interview that the “biggest crime” he committed against “the swamp” was joining with Ro Khanna, a Democrat from California, in writing a bill to release the Epstein files.

“That’s probably the only bill that’s passed Washington DC in the past 10 years that lobbyists haven’t written,” Massie said. “That’s when they decided that I had to be taken out – I was becoming effective so they wanted to eliminate me.”

He lost his Republican primary race this week to Ed Gallrein, in the most expensive US congressional primary election race in history. Gallrein was recruited by Trump to challenge Massie.

During Massie’s speech on Tuesday, he said that if the legislative branch always sides with the president, then “we do have a king”.

Massie now joins the ranks of Republicans who have broken with the president over the Epstein files and military action against Iran. Trump treated the primary race as a personal vendetta, calling Massie a “moron” and “loser”. He also traveled to Kentucky to campaign against Massie, calling him “disloyal to the United States of America”.

The president swiftly celebrated Massie’s defeat on Tuesday, declaring: “He was a bad guy. He deserves to lose.”