President Donald Trump endorsed Texas Attorney General Ken Paxton for Senate, dealing a massive political blow to longtime Sen. John Cornyn as he faces off against Paxton in an upcoming Republican primary runoff.

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Cornyn and Paxton advanced to a May 26 runoff after neither candidate won a majority of the vote in the March 3 primary, with the four-term senator narrowly ahead of Paxton, 42%-41%. The nominee will take on Democrat James Talarico in the fall.

Trump’s endorsement is the most powerful one there is in a GOP primary, and the president’s decision to back Paxton could put an end to Cornyn’s long political career, marked by more than two decades in the Senate following stints as state attorney general, state Supreme Court justice and a district court judge.

Trump’s endorsement, after early voting in the runoff began Monday, comes after weeks of speculation about whether the president would weigh in on the race. He had seemed ready to back Cornyn after the March primary, but he later tied his endorsement to passage of the SAVE America Act, a proposal overhauling the nation’s voting laws.

Paxton said he would consider dropping out of the race if Senate leadership agreed to nix the 60-vote threshold to end debate on legislation, known as the filibuster, in order to pass the measure.

Trump initially signaled that he wanted to avoid a divisive primary runoff, suggesting any candidate who he did not back should drop out of the race. But the deadline to avoid a runoff passed, and both Cornyn and Paxton’s names will be on the ballot, even if one of them ends his campaign.

Senate Majority Leader John Thune, R-S.D., publicly called on Trump to back Cornyn in the race. And Cornyn and allies have also made the case that he represents the party’s best chance at holding onto the Senate seat and avoiding any drag on the rest of the Republican ticket in Texas, given Paxton’s personal and professional controversies.

Paxton was impeached on bribery and corruption charges in 2023, though the state Senate acquitted him. Meanwhile, Paxton’s wife announced last year that she was divorcing Paxton on “biblical grounds.”

“I refuse to allow a flawed, self-centered, and shameless candidate like Ken Paxton risk everything we’ve worked so hard to build over these many years,” Cornyn said on March 3 after the runoff matchup became clear. “There is simply too much at stake in this midterm election for our state and for our country. The final two years of President Trump’s agenda hangs in the balance.”

Despite Cornyn and GOP leaders’ concerns, Trump told NBC News as he weighed his endorsement that he viewed both Cornyn and Paxton as candidates who could win in November. Trump called the Democratic candidate, Talarico, “so weak.”

Paxton, meanwhile, pitched himself as Trump’s truest ally in the race, and his supporters were quick to liken the embattled state attorney general to the president, who also faced two impeachments.

Paxton has criticized Cornyn’s work on a bipartisan gun bill after multiple mass shootings, including one at a Uvalde, Texas, elementary school, as well as Cornyn’s past criticisms of the president. In 2023, Cornyn said Trump’s “time has passed him by,” suggesting Trump wasn’t the most electable presidential candidate in 2024. He did eventually endorse Trump. Cornyn also said that the criminal charges alleging that Trump mishandled classified documents were “very serious.”

Paxton has also repeatedly touted his unwavering support for Trump.

“I want to tell you, the last time I felt this kind of energy in one room, I was actually at Mar-a-Lago when President Trump announced he was running again in 2024. I don’t know if you remember, but it was right after the midterms, and everyone was turning on the president, the establishment, the media and most Republican elected officials in America were lining up against him. Not me,” Paxton told his supporters on primary night.

Paxton later added that it was time to send Sen. Ted Cruz, R-Texas, and Trump “some reinforcements” in Washington.

“And I’m running to do just that,” Paxton said.