Another one bites the dust.

The past few weeks had been billed as a retribution tour for Donald Trump, as he settles old scores with his critics within the Republican party.

Call it what you will, but the evidence is now overwhelming that the Republican party is Trump's party and nothing - not an unpopular war in Iran, not sagging poll numbers among the general public, not rising consumer prices, not concerns about billion-dollar White House ballrooms - has changed that.

Thomas Massie, the independent-minded congressman from Kentucky, was comfortably defeated on Tuesday in the Republican primary race to decide who goes forward to take on the Democrats in November's midterms. The Trump-backed candidate Ed Gallrein is heading towards a 55% share of the vote.

Republican rebel Massie, a constant thorn in Trump's side, is just the latest in a political casualty count that is now stretching into double figures. His transgressions were multitude.

He opposed Trump's tax-and-spending budget package last year because he said it drove up the federal deficit. He voted to curtail the president's military operations in Venezuela and Iran. And, perhaps most notably, he was the driving force behind efforts in the House of Representatives to force the release of Justice Department files on Jeffrey Epstein, the disgraced financier and sex offender with ties to the rich and powerful.

All this put Massie front and centre on Trump's enemies list – and led to a $20m effort to oust him from the congressional perch he had occupied for more than a decade.

Trump once again proved his power in the Republican party, said Trey Grayson, a two-time Republican secretary of state in Kentucky.

Gallrien, Massie's handpicked Trump opponent, barely campaigned. He declined most debate and public forum invitations. He instead relied on the president's endorsement and millions of dollars of support it generated. That proved to be enough – and it wasn't all that close, as the challenger coasted to what appears to be a double-digit victory.

Massie now joins Louisiana Senator Bill Cassidy, who lost to a Trump-endorsed rival in a Republican primary, and five of seven Indiana state legislators opposed by the president last week among the ranks of the soon-to-be-unemployed politicians.

Next Tuesday, another incumbent senator, John Cornyn of Texas, may join them. Earlier on Tuesday, as Kentucky Republicans were still casting ballots, Trump endorsed his opponent, Texas Attorney General Ken Paxton, in next week's runoff election.

Trump's last-minute Paxton endorsement caught Republican senators by surprise – and prompted flashes of anger. Unlike Massie, who had few allies in Congress, Cornyn was a former member of his party's Senate leadership team.

Trump's willingness to target Republican incumbents may come at a cost. His polling numbers continue to show deep dissatisfaction with his performance on the economy, especially among independent voters. The loyal Republicans he has helped to victory may not be the best candidates in November to convince a wider electorate.

There is also the question of what these defeated Republicans do now in their final months. Earlier on Tuesday, Cassidy voted for the first time to support a resolution limiting Trump's authority to conduct the Iran war. He also expressed his opposition to providing a billion dollars in security funding for Trump's proposed White House ballroom.

There are a growing number of Senate Republicans who are leaving the chamber at the end of this year – either willingly or because they lost to Trump-backed opponents. They could make life difficult for the president in the coming months, as he tries to push through new spending packages or seeks to get administration nominees approved.

Trump is steadily removing his critics from within the Republican Party, but they may have a few parting gifts for the president on their way out.