Ken Paxton (R) defeated incumbent John Cornyn (R) in the Republican primary runoff for U.S. Senate in Texas on May 26, 2026.

Paxton and Cornyn were the top two finishers in the March 3 primary. The primary advanced to a runoff because neither received more than 50% of the vote. Cornyn led Paxton 42.0%–40.5% in the March 3 primary.

Before the runoff election, Axios' Nicole Cobler wrote that the runoff "extends one of the most consequential primaries in the country and sets up a bruising fight for the GOP Senate nomination." CNN's David Wright said "[m]any national Republicans and allies of Thune believe Cornyn is their safest bet on retaining the Senate seat in reliably red Texas. But Paxton, the three-term state attorney general, has a record of backing Trump – notably on his debunked claims of election fraud that preceded January 6 – and strong ties to the state Republican grassroots."

Writing in Politico, Liz Crampton said the primary results "showed some surprising strength for Cornyn, who had trailed Paxton in most public polls and whose allies were worried might finish far behind the MAGA firebrand...[indicating] the four-term senator still has a real chance to retain his seat in late May." Punchbowl News wrote that "the runoff dynamics favor Paxton. The electorate in a runoff is smaller and much more conservative. It’s the hard-core activist types who show up to vote the day after Memorial Day."

On May 19, 2026, President Donald Trump (R) endorsed Paxton. In a Truth Social post, Trump wrote: "Ken is a true MAGA Warrior who has ALWAYS delivered for Texas, and will continue to do so in the United States Senate. John Cornyn is a good man, and I worked well with him, but he was not supportive of me when times were tough."

Paxton was elected Texas Attorney General in 2014. He was also a member of the Texas House for ten years and of the Texas Senate for two. Birenbaum described Paxton's message in the runoff as "[arguing] that his grassroots support among the MAGA base of the party will make it easier for Republicans to turn out the lower-propensity members of Trump’s coalition who tend to stay home when the president is not on the ballot and counteract Democratic enthusiasm." U.S. Reps. Lance Gooden (R) and Troy Nehls (R) endorsed Paxton.

Cornyn was first elected to the Senate in 2002. He earlier served as state attorney general and on the Texas Supreme Court. Cornyn said he had delivered for Texas while in office and was running for re-election "so President Trump and I can pick-up where we left off." The Texas Tribune's Gabby Birenbaum described Cornyn's message in the runoff as "saying that Paxton would be laden down by his history of legal and personal baggage if he is the nominee, costing Republicans untold millions in the general election that could be spent in more competitive states." Senate Majority Leader John Thune (R) and the National Border Patrol Council endorsed Cornyn.

As of May 2026, The Cook Political Report with Amy Walter, Inside Elections with Nathan Gonzales, and Larry J. Sabato's Crystal Ball each rated the general election Likely Republican. In the 2024 election, incumbent Ted Cruz (R) defeated Colin Allred (D) 53%–45%. In 2020, Cornyn defeated M.J. Hegar (D) 54%–44%.

Thirty-three of the 100 U.S. Senate seats are up for election, and another two seats are up for special election. Democrats hold 13 of the seats up for election, and Republicans hold 22. As of April 2026, 11 members of the U.S. Senate announced they are not running for re-election.