Fresh off an endorsement from President Donald Trump, Rep. Julia Letlow of Louisiana is launching a U.S. Senate bid against fellow Republican and second-term Sen. Bill Cassidy.

Letlow announced her decision during a breakfast in Baton Rouge, according to three people with knowledge of the event who requested anonymity because they weren't authorized to comment publicly.

The development further complicates Cassidy's difficult reelection path and effectively marks the final failure of his attempts to placate the president after voting in 2021 to convict Trump on impeachment charges stemming from his efforts to overturn the 2020 election.

A reliable Trump vote in the House, Letlow was elected to the House in a 2021 special election after her husband died of COVID complications shortly after winning the congressional seat.

Trump's backing of Letlow pits the White House against the National Republican Senatorial Committee, which has endorsed Cassidy as part of its tradition of backing incumbents. The move also underscores Trump's alliance with Louisiana Gov. Jeff Landry, a firebrand conservative who months ago identified Letlow as an ideal challenger to Cassidy. Landry has since become Trump's envoy to Greenland as the president pushes for a U.S. takeover of the semi-autonomous territory of Denmark.

Cassidy, a physician who was once an outspoken critic of Republican health care policies before he entered politics himself, incensed Louisiana conservatives with his support for Trump's conviction in 2021. Once Trump emerged as the party's 2024 front-runner, Cassidy tried to return to the fold, with his official social media accounts consistently praising the president and casting him as a member of Trump's “Make America Great Again” movement.

As Senate health committee chairman, Cassidy provided the decisive vote to advance Robert Kennedy Jr.'s controversial nomination as health secretary, despite the senator expressing concerns about Kennedy's views on vaccines. Cassidy explained his vote saying he'd received assurances that Kennedy would not pursue certain changes to U.S. vaccine policy. Kennedy has proceeded to roll back U.S. vaccine recommendations.

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Lisa Mascaro contributed to this report.