Donald Trump displayed his supremacy over the Republican party on Tuesday when voters in northern Kentucky rejected the maverick congressman Thomas Massie in favour of the US president’s hand-picked challenger.

Ed Gallrein, a retired Navy Seal and farmer who was recruited into the race by Trump, defeated the seven-term incumbent in a primary election in Kentucky’s fourth congressional district in what the president’s allies framed as a test of whether dissent could still exist inside today’s Republican party.

The election took place as voters in five other states – Pennsylvania, Georgia, Alabama, Oregon and Idaho – went to the polls to decide their nominees for the November general election, in what was the biggest primary night of the year so far. Earlier on Tuesday, Trump endorsed Ken Paxton, the scandal-plagued Texas attorney general running for Senate, in a primary runoff against incumbent John Cornyn, infuriating some in his party.

In Kentucky, Massie now joins the ranks of Liz Cheney, Adam Kinzinger, Jeff Flake, Mitt Romney and other elected Republicans who were either ousted or decided to retire because of their party’s capitulation to Trump.

Over the weekend, Senator Bill Cassidy, who voted in favour of Trump’s conviction after the 6 January insurrection, lost a primary in Louisiana after the president backed challenger Julia Letlow.

Massie, a libertarian-minded conservative, repeatedly broke with the president over military action against Iran, government spending and the release of the Jeffrey Epstein files. He spent months insisting that Kentucky Republicans valued independence over obedience. Instead, voters in the deeply conservative fourth congressional district appeared to conclude that loyalty to Trump mattered more.

For months Trump had treated the contest as a personal vendetta.

He branded Massie a “moron”, a “nut job” and a “loser”, dispatched top advisers Chris LaCivita and Tony Fabrizio to run a super political action committee against him, and even travelled to Kentucky himself for a rally denouncing the congressman as “disloyal to the United States of America”.

Speaking to supporters on Tuesday evening, Massie said he had called Gallrein to concede.

“We’ve been honorable the whole time and we’re going to stay that way,” he said, issuing a plea for “basic decency” in politics.

In his remarks, Massie said that Tuesday marked six months since the passage of the Epstein Transparency Act, which forced the justice department to make public millions of documents related to its investigation into the late sex offender.

“We’ve taken out two dozen CEOs, an ambassador, a prince, a prime minister, a minister of culture – and that was just six months,” Massie said, listing some of the most high-profile resignations that resulted from the release of the files. Smiling, he added: “I’ve got seven months left in Congress.

Gallrein campaigned almost entirely as a loyal foot soldier for the president’s agenda. He accused Massie of suffering from “Trump derangement syndrome” and pledged to “stand shoulder to shoulder” with the White House.

The Hill website reported that Kentucky’s fourth congressional district became the most expensive House primary battle in history, citing figures from AdImpact that showed spending of $25.6m in television, radio and digital advertising.

“It’s not just the most expensive. This thing went on longer than Vietnam,” Massie said. “Why did this race get so expensive? Because they decided to buy the seat.”

Gallrein portrayed Massie as a politician who had drifted from Trump’s Make America Great Again movement despite benefiting from it politically for years. Maga Kentucky, the Super Pac backing Gallrein, flooded the district with attack ads accusing Massie of siding with Democrats and obstructing the president’s agenda.

Massie argued he was defending the very principles Trump once championed – opposition to endless wars, runaway deficits and government secrecy. But his message increasingly struggled to compete against the emotional force of Trump’s endorsement in the sprawling district, which stretches from Cincinnati’s southern suburbs to the Appalachian foothills.

Gallrein will now enter the general election as overwhelming favourite in a district that has not elected a Democrat in two decades.

Meanwhile, Trump-backed representative Andy Barr easily won a contested Republican primary for Senate in Kentucky to replace the long-serving former Senate majority leader Mitch McConnell, who is retiring.