Trump waded into race to defeat incumbent Bill Cassidy, a Republican and critic of Trump’s actions following the 2020 vote.

A candidate endorsed by United States President Donald Trump has won the Republican primary in Louisiana, putting her on track to win a seat in the US Senate in November.

Julia Letlow, currently a US representative, beat state Treasurer John Fleming in a two-candidate run-off on Tuesday.

While she will face Democrat Jamie Davis, a farmer, in the November midterm vote, Letlow will be the favourite in the southern state that skews heavily Republican.

Trump had first waded into the race ahead of the first round of primary voting on May 16. He endorsed Letlow while publicly opposing incumbent Senator Bill Cassidy.

Cassidy had been one of only a handful of Republicans to vote to convict Trump following his impeachment in connection with the January 6, 2021, storming of the US Capitol by his supporters.

A medical doctor, Cassidy has also clashed with the Trump administration over its health policies under Health and Human Services Secretary Robert F Kennedy Jr.

Still, Cassidy has largely supported Trump’s policy initiatives in recent years, in an apparent attempt to mend ties with the president.

He eventually finished third in the first round of voting behind Letlow and former Congressman Fleming, who have both been staunch supporters of Trump’s Make America Great Again (MAGA) movement.

Letlow’s victory underscores Trump’s continued hold over large portions of the Republican base.

He had successfully supported a challenger to incumbent Congressman Thomas Massie, another top Republican critic of the president, in Kentucky, and successfully backed loyalist Ken Paxton over incumbent Senator John Cornyn in Texas.

Trump has had less success in gubernatorial races, with his endorsees failing to win the Republican nominations for races in Iowa and Georgia.

It remains unclear if Trump’s involvement will prove advantageous in the general election, in which Democrats will try to wrest control of the US House of Representatives and the Senate from Republicans.

Polls have shown dipping support for Trump among the broader electorate. His approval rating has hit all-time lows amid the US-Israeli war with Iran, which has had knock on effects for the US economy.

His administration’s aggressive approach to deportations has also received low marks, particularly following the killing of two US citizens by immigration enforcement agents in January.

The political climate threatens to energise Democrats and potentially turn away voters unaffiliated with either party.

Some Republican party officials have also criticised Trump’s tendency to choose loyalty over the viability of a candidate.

For example, Republican party leaders have expressed particular dismay over Trump’s endorsement of Paxton over Cornyn in Texas.

Paxton, the state’s scandal ridden current attorney general, has been seen as a weaker candidate than moderate incumbent Cornyn in the general election.

He is now expected to face a punishing race against upstart Democratic opponent James Talarico, a Presbyterian seminarian who has found traction in the state with his blend of faith and liberal policy platforms.