A US senator from Louisiana bidding for a third term has lost his Republican primary, in the latest demonstration of US President Donald Trump’s hold over his party.

Trump had targeted Bill Cassidy and endorsed an opponent in retaliation for the senator voting to impeach him five years ago.

With 99 percent of votes counted in Saturday’s primary election, Cassidy was trailing in third place behind Trump-endorsed challenger Julia Letlow, a congresswoman, and state treasurer John Fleming, effectively shutting Cassidy out of a June 27 runoff.

“Congratulations to Congresswoman Julia Letlow on a fantastic race, beating an Incumbent Senator by Record Setting Numbers,” Trump posted on his Truth Social platform.

The 79-year-old president was also quick to celebrate Cassidy’s fall, writing: “His disloyalty to the man who got him elected is now a part of legend, and it’s nice to see that his political career is OVER!”

Cassidy delivered some pointed remarks in his concession speech. Without mentioning Trump by name, the lawmaker said American politicians, when they lose a race, don’t whine and “don’t claim that the election was stolen.”

“Our country is not about one individual, it is about the welfare of all Americans,” he said. “And if someone doesn’t understand that, and attempts to control others through using the levers of power, they are about serving themselves, they’re not about serving us.”

Trump on Sunday swiftly turned his attention to another major Republican nemesis in Congress, Kentucky’s Thomas Massie who is seeking an eighth term in the House of Representatives but faces a closely-watched primary against a Trump-endorsed challenger.