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.

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. He added that Massie had his enemies in the state politics – acquired by not playing nice with local business leaders and disregarding the everyday work of legislating in favour of high-publicity efforts. But in the end, it came down to yet another simple display of Trump's iron grip on the Republican party.

Gallrein, Massie's handpicked Trump opponent, barely campaigned and relied on the president's endorsement and millions of dollars of support 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 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 as Trump endorsed his opponent. Trump's willingness to target Republican incumbents may come at a cost, as polling shows deep dissatisfaction with his performance, particularly among independent voters. As Trump continues to remove critics from within his party, they could create obstacles for him in the coming months.