Rep. Julia Letlow announced her primary challenge to Sen. Bill Cassidy on Tuesday, just days after President Donald Trump lent his endorsement in the race, according to Politico.

The entrance of Letlow, a Louisiana Republican who has served in Congress since 2021, is a major blow to Cassidy, who was already fending off another MAGA-aligned challenger in a state Trump carried by 22 points in 2024.

“My parents taught me well. They taught me that when the Lord opens a door, you don’t walk through it — you run. It’s an honor to share with you today that I’m officially announcing my candidacy for the United States Senate,” Letlow said at an annual business event with other state leaders in Baton Rouge, Louisiana, according to two people who attended the event. 

Cassidy, who attended the event but left before Letlow’s announcement, said in a statement she had called him beforehand.

“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.”

Trump upended the primary in the Republican state over the weekend when he preemptively endorsed Letlow. “Should she decide to enter this Race, Julia Letlow has my Complete and Total Endorsement. RUN, JULIA, RUN!!!” Trump wrote on Truth Social.

Letlow said that she was “honored to have President Trump’s endorsement and trust,” but did not immediately launch her bid after receiving the president’s public backing.

Cassidy, seeking a third term in the Senate, has drawn criticism from Trump after voting to convict Trump in his 2021 impeachment trial over his role in the Jan. 6, 2021, Capitol riot. He has also clashed with the administration at times on health care, though he was a key vote to confirm Health and Human Services Secretary Robert F. Kennedy Jr. last year.

While Cassidy has maintained he will win, Trump’s endorsement of Letlow is a serious blow to his chances. Cassidy has told fellow GOP senators that top White House officials promised him they would stay neutral in the race, as POLITICO reported in September.

Trump’s endorsement — and Letlow’s entrance into the race — also adds a new complication for Senate GOP leadership, which has backed Cassidy.

Senate Majority Leader John Thune campaigned with Cassidy in Lousiana late last week and privately encouraged Trump to support him during a call Friday. And Sen. John Barrasso of Wyoming, the No. 2 GOP leader, said Tuesday he has been supporting Cassidy “from the beginning.”

“We’re going to do everything we can to build the majority,” Barrasso said when asked how much of a “headache” Trump’s endorsement is for Republicans.

Letlow first entered Congress after her husband, Rep.-elect Luke Letlow (R-La.), died of COVID before entering office, and in her Tuesday speech compared her career trajectory to that of former Rep. Lindy Boggs (D-La.), who won her House seat after her husband, Rep. Hale Boggs (D-La.), died in office.

In office, Letlow has been a strong ally of both Speaker Mike Johnson and Majority Leader Steve Scalise, Louisiana’s most powerful House members.