A Republican senator who broke from his party to vote in favour of convicting US President Donald Trump in impeachment proceedings during his first term is facing a bruising primary challenge in his home state of Louisiana.

Bill Cassidy’s primary race on Thursday has been seen as a barometer of Trump’s continued hold over the Republican Party. Even as polls have shown the president’s approval tanking, early primary votes have shown the continued weight his endorsement carries.

Trump has backed US Representative Julia Letlow in the Senate race. State Treasurer John Fleming is also running. The winner of the Republican primary is all-but-assured to win in the general election in the deep-red state.

Cassidy had joined seven Republicans in the Senate in voting to convict Trump of “incitement of insurrection”, following his campaign to overturn the 2020 election results and his supporters’ storming of the US Capitol on January 6, 2021.

“Our Constitution and our country is more important than any one person. I voted to convict President Trump because he is guilty,” Cassidy said in a statement at the time.

Despite the handful of Republican defections, the chamber fell far short of the two-thirds majority needed to convict Trump of the charges, of which he was acquitted.

Initially viewed as politically toxic after leaving office in 2021, Trump mounted a stunning comeback in the years that followed, reshaping the Republican Party in his likeness.

That included the ascension of many lawmakers who endorsed Trump’s claims that the 2020 vote was stolen, for which he has provided no evidence.

Currently, most other Republican senators who voted to convict Trump alongside Cassidy have been ousted or chosen to leave office.

Among the group, only Republican centrists Susan Collins from Maine, who continues to be seen as a bulwark against Democratic challengers in her home state, and Lisa Murkowski from Alaska, who saw off a Trump-backed challenger in 2022, have escaped major intra-party fallout for their votes.

Letlow, an academic administrator who entered office in 2021, has also seized on Cassidy’s 2021 vote, saying in her campaign launch video that residents of Louisiana “shouldn’t have to wonder how our senator will vote when the pressure is on”.

A fine line

Cassidy, a physician, has walked a fine line during Trump’s second term, regularly touting the administration’s policy initiatives and appearing alongside Trump at the White House several times for healthcare-focused events and bill signings.

Still, Cassidy has had some high-profile clashes with the Trump administration. During Robert F Kennedy Jr‘s confirmation hearing to become health and human services secretary, Cassidy sparred with Kennedy over his vaccine scepticism.

“I am a doctor who has seen people die from vaccine-preventable diseases, and when I see outbreaks numbered in the thousands, and people dying once more from vaccine-preventable diseases, particularly children, it seems more than tragic,” he said during the hearing.

Cassidy later cast the deciding vote to confirm Kennedy after receiving assurances that he would not change federal vaccine recommendations. The HHS under Kennedy has since changed those recommendations.

In April of this year, Trump accused Cassidy of tanking his nominee for surgeon-general, Casey Means, who had come under fire for her vaccine scepticism and unproven wellness theories.

Trump decried what he called Cassidy’s “intransigence and political games”. In a subsequent post, he said hopefully Republicans “will be voting Bill Cassidy OUT OF OFFICE in the upcoming Republican Primary!”

Cassidy, in turn, has claimed opponent Letlow does not have conservative bona fides.

He has highlighted her past support of education diversity initiatives, which she has since disavowed, as well as her past attendance at the 2023 United Nations climate change conference.

Trump’s sway?

Trump carried Louisiana in the 2016 and 2020 presidential elections with about 58 percent of the vote, and in 2024 with 60 percent.

Heading into the primary vote, the president’s overall national approval rating has tanked, hitting a record low of 34 percent at the end of April. That has come amid widespread discontent over the US-Israel war on Iran and its economic toll.

Trump has maintained strong support among Republicans, but has notably seen dipping support among independents.

Polls have shown Cassidy trailing behind both Letlow and Fleming. If no candidate wins an outright majority, the race will move to a run-off on June 27.

Thursday’s race takes place amid an ongoing national battle over congressional redistricting.

While Louisiana’s US House of Representatives primary was also scheduled for Thursday, Governor Jeff Landry has temporarily suspended the vote.

That after the US Supreme Court struck down a major provision of the Voting Rights Act, paving the way for the state’s Republican-controlled legislature to redraw its congressional map to do away with one of two Black-majority districts.

Civil rights groups have filed a lawsuit alleging the suspension violates both the US and the state’s constitutions.