GOP Texas Senate Candidate Ken Paxton speaks to supporters at a watch party on March 3, 2026 in Dallas, Texas. Paxton and incumbent John Cornyn will face off again in a run off.Sergio Flores | Getty ImagesPresident Donald Trump endorsed Texas Attorney General Ken Paxton in Texas' Republican Senate primary runoff Tuesday, throwing his weight behind a hard-line ally and dealing a major blow to incumbent Sen. John Cornyn one week before the state's Election Day.The endorsement gives Paxton a major boost in one of the GOP's most closely watched Senate primaries, a race that has become a test of Trump's grip on Republican voters and the strength of the Senate GOP establishment."Ken is a true MAGA Warrior who has ALWAYS delivered for Texas, and will continue to do so in the United States Senate. He will tirelessly fight to continue the Great Growth of our Economy, Cut Taxes and Regulations," Trump said in a Truth Social post. "Ken Paxton has gone through a lot, in many cases, very unfairly, but he is a Fighter, and knows how to WIN. Our Country needs Fighters, and also Loyalty to the Cause of Greatness."The May 26 runoff pits Paxton, a longtime Trump ally, against Cornyn, a four-term incumbent who has served in the Senate since 2003. Early voting began Monday and runs through Friday."I am incredibly honored to have President Trump's COMPLETE AND TOTAL ENDORSEMENT," Paxton posted on X following the endorsement. "No one has ever fought harder for the American people than President Trump, and I look forward to championing his America First agenda in the Senate!"Sen. John Cornyn, R-Texas, speaks to the press, acknowledging he will face a run-off with Texas Attorney General Ken Paxton in the race for U.S. Senate after the Texas Republican primary election, March 3, 2026 at the Austin Marriott Downtown in Austin, Texas. Sarah Diggins | Austin American-statesman | Hearst Newspapers | Getty ImagesTrump's influenceThe Texas race is the latest example of Trump flexing his influence in Republican primaries as he moves to punish lawmakers he views as insufficiently loyal and reward candidates aligned with his political movement."John Cornyn is a good man, and I worked well with him, but he was not supportive of me when times were tough and, despite having the Most Successful Economy in the History of our Country during my First Term and, with all of the many other things that I accomplished," Trump posted.In response, Cornyn posted on X that he has "worked closely with President Trump through both of his Presidential terms and voted with him more than 99% of the time.""He has consistently called me a friend in this race. It is now time for Texas Republican voters to decide if they want a strong nominee to help our GOP candidates down ballot and defeat Talarico in November, or a weak nominee who jeopardizes everything we care about. I trust the Republican voters of Texas," Cornyn continued, referring to Democratic nominee James TalaricoIt is the third major GOP primary race in a short span where Trump has exerted his influence. Trump helped end the reelection bid of Sen. Bill Cassidy, a Louisiana Republican who voted to convict him in his second impeachment, and is backing Ed Gallrein, a former Navy SEAL challenging Rep. Thomas Massie, R-Ky., in Tuesday's Republican primary. Massie has frequently broken with Trump and House GOP leadership. Those contests have become referendums on Trump's hold over the Republican Party ahead of the 2026 midterms. While Trump's overall approval ratings have weakened, he remains deeply popular with Republican voters, giving his endorsements outsize power in primaries.Senate Republicans warned that Paxton could be a more difficult and expensive general election candidate than Cornyn.Sen. Lindsey Graham, R-S.C., said Trump's endorsement of Paxton could make the race "three times more expensive" for Republicans."I like Sen. Cornyn," Graham told reporters at the Capitol. "If Paxton wins the primary, I will be for him. But you don't have to be a rocket scientist to figure out the pathway for Paxton is there, but it's more uphill. And it will cost more."Sen. Lisa Murkowski, R-Alaska, was blunter, calling Cornyn "a terrific senator" who has worked with Trump's agenda."The fact that the president would choose to endorse not Sen. Cornyn but a candidate who probably is going to struggle mightily in the general is a problem," Murkowski said.Texas Senate candidate James Talarico (D-TX) speaks at a campaign rally on March 2, 2026 in Houston, Texas. Talarico is visiting various locations around the state in the lead up to tomorrow's primaries. Danielle Villasana | Getty ImagesThe general electionIn the final stretch leading up to Election Day, Cornyn touted his support for Trump, writing Monday on X that he has a 99.3% voting record with the president. Cornyn has leaned on the backing of Senate Republican leadership and outside allies, who have spent more than $60 million to help protect him, according to Reuters.Paxton, meanwhile, has sought to cast Cornyn as a creature of the Washington establishment, while Cornyn has attacked Paxton as unfit for office, pointing to his years of legal and political controversy, including his 2023 impeachment by the Texas House. Paxton was later acquitted by the Texas Senate.Polling shows a neck-and-neck race between the two men. A statewide survey from the University of Houston Hobby School of Public Affairs, conducted from April 28 to May 1, found Paxton leading Cornyn 48% to 45% among likely runoff voters, slightly outside the poll's margin of error.The winner of the runoff will face Talarico in November. Talarico, a 37-year-old state representative, defeated Rep. Jasmine Crockett in the March Democratic primary and has posted strong fundraising numbers, including more than $27 million raised in the first quarter, according to his campaign."We already know who we're running against: the billionaire mega-donors and their corrupt political system. For decades, John Cornyn and Ken Paxton have embodied a broken politics that enriches wealthy donors while costs skyrocket for the rest of us," Talarico said in a statement after the endorsement. "Our movement to take back Texas for working people rises above party politics — because the biggest fight in this country is not left versus right, it's top versus bottom," Talarico said. Republicans are still favored to hold the Texas seat, but Democrats have pointed to the bruising GOP primary as a potential opening. A Texas Southern University poll released Monday found Talarico in close hypothetical matchups against either Republican: Cornyn led Talarico 45% to 44%, while Paxton and Talarico were tied at 45%.Trump won Texas by nearly 14 percentage points in 2024. —Justin Papp contributed to this report.