Donald Trump has endorsed the Texas attorney general, Ken Paxton, in the state’s Republican primary, bolstering his bid to unseat the incumbent US senator, John Cornyn.

The US president praised Paxton, a hardliner who has pitched himself as a political warrior for Trump’s Make America Great Again movement, as an “America First Patriot” in a post on social media on Tuesday.

The intervention, which comes ahead of next week’s May primary runoff, comes despite warnings from other leading Republicans that Cornyn would be a stronger candidate in November’s general elections.

Democrats have nominated the Texas state representative, James Talarico, as their candidate for Senate. Recent polls have put Talarico, with his blend of faith-based populism, bipartisan appeal and generational energy, in a tight race with Cornyn and Paxton.

“Ken is a true MAGA Warrior who has ALWAYS delivered for Texas, and will continue to do so in the United States Senate,” Trump wrote on Truth Social.

But Paxton has been embroiled in a series of controversies. He was acquitted in a 2023 impeachment trial on corruption charges. He also reached a deal in 2024 to end a long-running securities fraud case.

Cornyn, an old guard Republican, and Paxton face a 26 May runoff after neither secured a majority in their three-way March primary election.

Both candidates had tried to position themselves as closely to Trump as possible. Cornyn introduced legislation last week to rename a highway after the president, declaring that “nearly 1,800 miles of open road” would “forever be known as the Trump Interstate”.

“I am incredibly honored to have President Trump’s COMPLETE AND TOTAL ENDORSEMENT,” Paxton wrote on social media on Tuesday.

Cornyn “is a good man, and I worked well with him, but he was not supportive of me when times were tough”, wrote Trump, adding that the US senator – who has represented Texas in Washington since 2002 – was “very late” in backing his run for the White House.

The prevailing Republican candidate will come up against an ambitious campaign by Democrats to flip the seat, in a reliably Republican state.

Talarico has emerged as a rising star in the Democratic party, and an effective fundraiser: his campaign said it raised $27m in the first three months of 2026, the largest-ever sum for a US Senate candidate in the first quarter of an election year.

Cornyn said: “It is now time for Texas Republican voters to decide if they want a strong nominee to help our GOP candidates down ballot and defeat Talarico in November, or a weak nominee who jeopardizes everything we care about.”

In a statement, Talarico said it “doesn’t matter” who wins the Republican run-off. “We already know who we’re running against: the billionaire mega-donors and their corrupt political system,” he said.

Texas congresswoman Jasmine Crockett, a Democrat who lost the party’s nomination for US Senate to Talarico, said she was a “bit speechless” when informed by the Guardian at an event in south LA on Tuesday morning that Trump had endorsed Paxton.

“Ken Paxton is a disaster,” said Crockett, speaking from Dulan’s soul food restaurant, where she endorsed HHS secretary Xavier Becerra for governor of California. “This is the crème de la crème of the Republican Party nowadays.”

Asked about Trump’s decision to weigh into the race, Vice-President JD Vance suggested that being “out of step” with Trump was “not a good place to be, politically”.

“I’ve known John Cornyn a long time,” Vance told the White House press briefing. “But unfortunately, when it really counted, Ken Paxton was there for the country and for the president, and that’s why he ultimately earned the president’s endorsement.”

Reuters contributed reporting