A Trump-backed challenger has defeated a two-term Republican senator who voted to convict the president at his 2021 impeachment trial.

Congresswoman Julia Letlow will advance to a runoff next month, ousting incumbent Bill Cassidy, who President Donald Trump branded a "disloyal disaster" ahead of Louisiana's high-stakes contest on Saturday.

State treasurer John Fleming, another Trump-aligned candidate, also advanced to the Republican runoff for Louisiana's Senate seat.

The top two candidates, Letlow and Fleming, will face off again in late June as neither won a simple majority. The candidate who wins the runoff will then run against a Democratic candidate in the general election.

Cassidy, 68, was one of seven Senate Republicans who voted to convict Trump when the president was impeached after the 6 January attack on the Capitol in 2021. Trump was acquitted after the Senate voted 57-43, short of the two-thirds majority required.

Of the seven Republicans who voted to convict him, only three still serve in the Senate: Lisa Murkowski of Alaska, who survived a primary challenge in 2022; Susan Collins of Maine; and Bill Cassidy.

During his re-election campaign, the Louisiana senator sought to repair his strained relationship with Trump.

"I don't really think President Trump likes me that much, but we work really well together," Cassidy told reporters last week, pointing to several bills he sponsored that were later signed into law by the president.

But Trump had made it clear he wanted Cassidy gone, and in January encouraged Letlow, 45, to challenge the senator.

"I want to say thank you to a very special man - the best president this country has ever had, President Donald Trump," Letlow said in a speech after the late evening results.

In his election night remarks, Cassidy did not mention Trump by name. But he did allude to the president and his false claims that the 2020 election he lost had been stolen.

"When you participate in democracy, sometimes it doesn't turn out the way you want it to," Cassidy told supporters in Baton Rouge. "But you don't pout. You don't whine. You don't claim that an election was stolen from you."

American leaders, he said, should not be focused on loyalty to one individual, but rather to the wellbeing of the general public.

"And if someone doesn't understand that and attempts to control others through using the levers of power, they're about serving themselves," he said. "They're not about serving us. And that person is not qualified to be a leader."

Trump cheered the results on Truth Social, his social media platform, writing, "it's nice to see that his political career is OVER!"

"That's what you get by voting to Impeach an innocent man," the president added in another post.

With the contest now headed to a runoff, it remains unclear whether Trump will stay actively engaged in the race.

Letlow, who became the first Republican woman elected to represent Louisiana in Congress in 2021, won about 45% of the vote in the primary, US media report.

Fleming, her rival in the June primary runoff, previously was a representative for Louisiana in Congress and also worked in the first Trump administration.

He narrowly beat Cassidy with around 28% of the vote, compared to Cassidy's 25%, according to US media.

"Yesterday, the people of Louisiana proved that a grassroots conservative can still compete, and win, even when the political establishment and outside dark money groups spend millions of dollars trying to destroy him," Fleming said in a statement on Sunday.