Representative Julia Letlow of Louisiana said on Tuesday that she was officially running for Senate, challenging Senator Bill Cassidy in a Republican primary contest days after President Trump said he would back her.
“RUN JULIA RUN!!!” Mr. Trump wrote in a social media post on Saturday night.
Ms. Letlow made her announcement at a breakfast of business leaders on Tuesday, according to a report in the Greater Baton Rouge Business Report, and she later posted a video online.
“I am grateful for the courageous leadership of our president,” she said. “I’ve witnessed firsthand how President Trump is taking on the swamp and delivering for the American people.”
Mr. Cassidy said in a statement that Ms. Letlow had called him to share the news.
“She said she respected me and that I had done a good job,” he said. “I will continue to do a good job when I win re-election. I am a conservative who wakes up every morning thinking about how to make Louisiana and the United States a better place to live.”
Ms. Letlow entered Congress in 2021 after her husband died from the coronavirus weeks after he won a congressional race in 2020. She ran in a special election.
Mr. Cassidy already faces several other primary challengers, including John Fleming, the Louisiana treasurer and a former congressman.
Mr. Cassidy has been an occasional critic of Mr. Trump, but he most notably voted to convict the president in his second impeachment trial five years ago. Years after Mr. Trump pushed during the 2022 midterm elections to excise House Republicans who voted to impeach him, he is now trying to punish Mr. Cassidy in the first election the senator has faced voters since his impeachment trial vote.
Ms. Letlow made an oblique reference to this in her kickoff video, saying, “We shouldn’t have to wonder how our senator will vote when the pressure’s on.”
Mr. Trump has for months privately mulled backing Ms. Letlow. The New York Times reported over the weekend that Mr. Trump first met with her in March 2025 and said that he would support her but that he would lie low to avoid a repeat of the president’s clashes with Mitt Romney, a former senator who routinely and publicly clashed with Mr. Trump.
Mr. Cassidy, a physician, had made some attempts to make amends with Mr. Trump, most notably voting last year to confirm Robert F. Kennedy Jr. as health secretary, despite the senator’s advocacy for vaccines.
The primary election is on May 16. Mr. Cassidy had $9.5 million in his campaign accounts as of October, while Ms. Letlow had $2.3 million.
Ms. Letlow’s candidacy sets off the second major challenge of a sitting Republican senator from the right this year. In Texas, Senator John Cornyn is in a three-way race with Ken Paxton, the state attorney general, and Representative Wesley Hunt.
Shane Goldmacher is a Times national political correspondent.
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