With two days to go before the next big test of Donald Trump’s iron grip over his party, the president went head-to-head on Sunday with his nemesis, Thomas Massie the Kentucky congressman who is in a fight for his political life in Tuesday’s Republican primary.

Over an eight-hour period starting in the early hours of Sunday, Trump took to his bully pulpit on Truth Social to taunt Massie, one of very few senior Republicans who has dared to defy him. Massie is the “worst and most unreliable Republican Congressman in the history of our Country”, the rant began, followed by a mid-morning exhortation to Kentucky voters to “vote the bum out on Tuesday”.

Massie has been a consistent thorn in Trump’s side, voting against his signature tax and spending cuts bill, helping to force the justice department to release the Jeffrey Epstein files, and insisting on congressional oversight over the military actions in Venezuela and Iran. Now he faces a bruising primary against his Trump-endorsed challenger, Ed Gallrein.

Undeterred by Trump’s lurid attacks, Massie put on a brave face on the Sunday political show This Week on ABC News. “I’m the only one they haven’t been able to bully,” he said, adding: “I’m ahead in the polls, and they’re desperate … That’s why the president’s losing sleep and tweeting about this.”

In fact, Massie’s position is not as comfortable as he made out. An independent poll by Quantus Insights last week put Gallrein, a farmer and retired US Navy Seal, ahead of the incumbent representative 48% to 43%, with 8% undecided.

Massie insisted on CNN that he was buoyed up by the support of anti-abortion and gun rights groups in the state, and by millions of dollars in donations from grassroots voters. By contrast, he blamed super-wealthy donors including Miriam Adelson and Paul Singer and what he called the “Israeli lobby” for bombarding Kentucky with money to unseat him.

The prevailing political wind is certainly in Trump’s direction, as he continues to stamp out any sign of autonomous thought within his party. On Saturday Bill Cassidy, the Republican senator from Louisiana, was ousted in a primary as punishment for having voted to convict Trump in his 2021 impeachment over the January 6 insurrection.

Julia Letlow, a US representative endorsed by Trump, and the state treasurer John Fleming both surpassed Cassidy and will now compete in a runoff election on 27 June.

Though Cassidy did not mention the president by name, he left little to the imagination in his concession speech. “If someone attempts to control others through using the levers of power, they’re about serving themselves, they’re not about serving us, and that person was not qualified to be a leader,” he said.

Cassidy’s ejection means that of the seven Republican senators who voted to convict Trump in the January 6 impeachment trial, only two remain in place – Susan Collins, who is in a tough re-election fight in Maine, and Lisa Murkowski from Alaska. Only one of the 10 Republican House members who voted to impeach Trump in 2021 are left running for re-election in November: David Valadao from California.

“The headline is: Trump strong,” said Lindsey Graham, the Republican senator from South Carolina, on NBC News’s Meet the Press. “Bill Cassidy lost because he tried to destroy Trump, Massie is going to lose because he tried to destroy the agenda. If you try and destroy him, you are going to get destroyed – that’s the takeaway.”

Mike Johnson, the Republican speaker of the US House, issued the same effective threat to would-be dissenters in his party in an interview with Fox News Sunday, though he cast it in more fluffy language. Trump’s impact on elections across the country showed that he wielded “the most powerful endorsement in the history of politics”.

Trump’s enduring ability to spread fear across his party and effectively silence any internal opposition runs counter to his overall standing with the American people. The US-Israel war with Iran, and the consequent economic fallout including rising fuel prices, are taking their toll.

A new CBS News poll found that approval of Trump’s handling of inflation among Republicans has fallen from 74% in March to 63%. When asked how they felt about Trump’s approach to the economy, 70% of all respondents said they were frustrated or angry.

The combination of Trump’s unpopularity among American voters as a whole and his rigid control of his party is being read as a potential opportunity by Democrats. Pete Buttigieg, the former US transportation secretary under Joe Biden and a possible 2028 presidential contender, told CNN’s State of the Union that he saw a “big opening for Democrats”.

Under Trump’s unbending control, “we are seeing more and more extreme candidates put forward in their House and Senate races,” Buttigieg said. “The Republican party is organized less and less around conservative principles, more and more around one man, and as that one man remains deeply unpopular they’re finding they’re having a very hard time convincing the rest of America to vote for them.”