President Donald Trump on Tuesday endorsed Texas Attorney General Ken Paxton in his primary challenge to Sen. John Cornyn, dealing a massive blow to the incumbent with major implications in the fight for the Senate majority.

Cornyn has spent over a year courting Trump’s support, while Paxton has run to Cornyn’s right and pitched himself as a stronger ally of the president. Cornyn and Senate GOP leaders argued that he is a stronger general-election candidate as Democrats look to win statewide office in Texas for the first time since 1994.

The runoff is a week away, and early voting began Monday.

“I know Ken well, have seen him tested at the highest and most difficult levels, and he is a WINNER!” Trump said in a lengthy Truth Social post.

“John Cornyn is a good man, and I worked well with him, but he was not supportive of me when times were tough,” he added, appearing to reference how Cornyn hesitated to support Trump in his 2024 campaign.

Cornyn said in the spring of 2023 that he thought Trump’s “time has passed him by,” though the Texas senator eventually endorsed the president after he won the New Hampshire primary.

Trump’s intervention in the runoff comes at a high-profile moment for his dominance within the GOP. Earlier this month, he helped unseat a group of Indiana state lawmakers who resisted his push for mid-decade redistricting. On Saturday, Louisiana Sen. Bill Cassidy lost his primary, five years after he voted to convict Trump in his second impeachment. And on Tuesday, Trump is working to defeat one of his biggest GOP critics in Congress — Rep. Thomas Massie of Kentucky — in his primary.

Democrats have been eyeing the Texas Senate seat in their uphill battle to capture the chamber majority, and the state becomes particularly enticing if the scandal-plagued Paxton defeats Cornyn. The state’s Democrats resolved their primary on March 3, picking a well-funded standard-bearer in state Rep. James Talarico.

While Trump stayed out of the GOP primary, which also featured Rep. Wesley Hunt, he quickly raised expectations for an endorsement in the runoff.

The morning after Cornyn finished narrowly ahead of Paxton in the primary, Trump said he would make an endorsement “soon” and that he expected the other candidate to drop out. But Trump was frustrated that his plans to endorse Cornyn were shared with the press, and then stayed neutral for most of the runoff — signaling he had cooled on intervening in the intraparty battle.

In teasing his endorsement earlier Tuesday, Trump told CNN’s Alayna Treene he had “pretty much always known who I was going to endorse. I just thought this was a good time.”

Paxton said on X that he was “incredibly honored” to have Trump’s endorsement.

Appearing on “The Charlie Kirk Show” a short time after Trump promised an imminent endorsement, Paxton said he trusted the president to “make a good decision.”

“We all know that Donald Trump’s endorsement is the most significant endorsement in the country and maybe the most significant endorsement in my lifetime,” Paxton said.

Paxton, who has been attorney general since 2015, has made a political career out of his loyalty to Trump. He sued four battleground states to overturn Trump’s 2020 reelection loss — the Supreme Court declined to hear the case — and spoke at the rally that preceded the January 6, 2021, attack on the US Capitol.

Paxton was among the first elected officials in the country to back Trump’s 2024 comeback campaign, as well attended Trump’s criminal trial in New York to show his support.

Paxton has continued to persevere in Texas politics despite a long list of scandals. He has battled state and federal securities fraud charges that stem from investment activities before he was attorney general. The Texas House impeached him in 2023 on charges that he abused his office, but the state Senate acquitted him after a trial. And he has faced multiple allegations of infidelity, including from his wife, state Sen. Angela Paxton, who filed for divorce last year.

Paxton has often portrayed his legal and ethical challenges as politically motivated, though the divorce has proven harder to downplay and factored heavily into Cornyn’s advertising against Paxton.

This story has been updated with additional details.