President Trump on Saturday night endorsed a potential primary challenger to Senator Bill Cassidy of Louisiana, an occasional Republican critic who voted to convict the president in his second impeachment trial, in a surprise announcement that threatened his relationship with Senate Republicans.
Mr. Trump said in a Truth Social post that he was backing Representative Julia Letlow of Louisiana, who won a special election in 2021 after her husband died from the coronavirus after his election but before he was sworn in. Ms. Letlow has not yet entered the Senate race, but Mr. Trump said that if she decided to do so, he would be with her all the way.
“RUN JULIA RUN!!!” Mr. Trump wrote, adding that “Should she decide to enter this Race, Julia Letlow has my Complete and Total Endorsement.”
The move is likely to create new tensions between Mr. Trump and Senate Republicans.
Mr. Trump called Senator John Thune of South Dakota, the majority leader, on Friday and said he was likely to endorse Ms. Letlow, according to a person briefed on the conversation who was not authorized to discuss the private remarks.
Mr. Thune pushed him to support Mr. Cassidy, the person said, noting potential consequences for key votes in the future. The majority leader also noted that Mr. Trump was starting to push a major health care bill, which Mr. Cassidy would have jurisdiction over as chairman of the Senate’s health committee.
Mr. Cassidy responded to Mr. Trump in a social media post of his own on Saturday night: “I’m proudly running for re-election as a principled conservative who gets things done for the people of Louisiana. If Congresswoman Letlow decides to run I am confident I will win.”
Ms. Letlow did not respond to a request for comment on Saturday night, but on social media, she wrote that she was honored by the endorsement and made it clear that she was likely to announce a campaign.
“This United States Senate seat belongs to the people of Louisiana, because we deserve conservative leadership that will not waver,” she wrote.
The timing of Mr. Trump’s endorsement surprised Ms. Letlow, according to a colleague who spoke to her on Saturday night. The congresswoman had told people she would not enter the race without Mr. Trump’s official backing. But the move had long been in the works.
Mr. Trump first met with Ms. Letlow on March 5 of last year, the morning after the president’s address to a joint session of Congress. The Oval Office meeting included, among others, Susie Wiles, the White House chief of staff; James Blair, a deputy chief of staff; and Representative Lance Gooden of Texas. Mr. Gooden helped arrange the meeting with Ms. Letlow.
In the meeting, Mr. Trump told Ms. Letlow that he would support her if she decided to challenge Mr. Cassidy, according to a person who was in the room and insisted on anonymity to describe the private conversation. But he said he would have to lie low for the time being to avoid a repeat of Mr. Trump’s clashes with former Senator Mitt Romney of Utah, who routinely broke with the president, including on his vote to remove him from office.
Ms. Letlow, a former teacher whose political profile in her state grew when redistricting added Baton Rouge to her mostly rural constituency, is not known in the Capitol as an aggressive politician. She was hesitant to make any moves with only private assurances of Mr. Trump’s support.
Ms. Letlow spoke again with the president at the congressional picnic on June 12.
He again privately expressed his support, according to a person who heard their conversation. But he had still said nothing publicly. And she had still heard nothing more concrete from Mr. Trump or anyone in his political operation as of Dec. 11, when she announced her engagement at a holiday ball at the White House.
Big donors, meanwhile, were still giving money to Mr. Cassidy and telling Ms. Letlow that, until she formally declared that she was running, they would support him.
Ms. Letlow had almost resigned herself to staying out of the race. This month, she filmed two campaign commercials: one for a re-election campaign to the House, and one for a potential Senate race. As the February filing deadline approached, she did not know which one she would be using, according to a person briefed on her campaign plans.
But Ms. Letlow has been building quiet support from encouraging colleagues in the House, who said they expected her to jump in now and campaign aggressively.
Any endorsement of a candidate opposing a sitting senator is viewed as unwelcome by Senate Republicans, and it is a move the conference writ large has sought to avoid. That includes Mr. Cassidy, who voted in favor of Mr. Trump’s choice for health secretary, Robert F. Kennedy Jr., last year, despite expressing misgivings.
Mr. Cassidy, who is a physician and a proponent of vaccines, has remained a public skeptic of Mr. Kennedy.
But he has taken pains to repair relations with Mr. Trump. As of late Saturday, the post he had pinned atop his X account featured a photograph of a meeting with Mr. Trump in the Oval Office as the president signed red hats.
“This is what a great working relationship looks like,” the October post from Mr. Kennedy said.
Mr. Trump’s endorsement of a potential Cassidy challenger comes days after the president frustrated Senate Republicans when he declared that five other G.O.P. senators should never be re-elected because of their opposition to him on military intervention in Venezuela. Among them was Senator Susan Collins of Maine, who is seen as the party’s most vulnerable incumbent in 2026.
Mr. Trump has also been unwilling to throw his support behind Senator John Cornyn, Republican of Texas, who is facing a competitive primary race against two challengers, Ken Paxton, the state attorney general, and Representative Wesley Hunt of Texas.
Mr. Thune indicated to reporters last week that, despite his best efforts to persuade the president to back Mr. Cornyn, the three Texans were going to have to “duke it out in the primary.”
The National Republican Senatorial Committee did not immediately respond to a request for comment on Mr. Trump’s latest move.
Alex Latcham, the executive director of the main super PAC aligned with Senate Republicans, said in a statement on Saturday that his group’s mission was “to preserve and expand” the Republican Senate majority.
“Anything that distracts from our efforts to beat Democrats in November is unhelpful,” he said.
Annie Karni is a congressional correspondent for The Times.
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