Trump-backed Texas Attorney General Ken Paxton will win the Senate primary runoff for the GOP nomination, CBS News projects, unseating longtime incumbent Sen. John Cornyn in a seismic shift for Republicans in the Senate.

Paxton will face the Democratic nominee, state Rep. James Talarico, in November. A Democrat has not won a statewide race in Texas since 1994, and has not won a U.S. Senate race in the state since 1988. 

A last-minute endorsement by President Trump last week boosted Paxton's fortunes. At his victory party on Tuesday, Paxton thanked Mr. Trump for his support, saying, "when everyone in Washington told him to abandon me and abandon the people of Texas, he didn't listen." 

"President Trump is the leader of our party, and his endorsement is the most powerful force in politics," Paxton said. 

Mr. Trump congratulated Paxton on his win, posting on social media that Cornyn ran "a strong and powerful race" and added that Cornyn "had a truly great career." 

"John will remain my friend for a long time to come, as we both watch Ken become a fantastic, common sense Senator, one who is respected by all," Mr. Trump said.

Cornyn said at his election night event on Tuesday that he would support the Republican ticket in the general election.

"I've said throughout this race that I trust the voters of Texas, and they made their decision, and I must respect it," Cornyn said.

Cornyn, a four-term incumbent who once served in Senate GOP leadership, had received the most votes in a three-way primary race with Paxton and Rep. Wesley Hunt in March, but the contest advanced to a Paxton-Cornyn runoff after no candidate won a majority. 

After previously staying on the sidelines, Mr. Trump threw his support behind Paxton last week, writing that the attorney general "has always been extremely loyal to me." The president called Cornyn a "good man" who "was not supportive of me when times were tough. Hunt also backed Paxton after the primary. 

The race is the most expensive primary in history. The Senate GOP campaign arm had invested millions in the race to defeat Paxton, and Senate Republicans had believed it would be easier to defend the seat against Talarico with Cornyn at the top of the ticket. 

Texas' other elected officials, including U.S. Sen. Ted Cruz and Gov. Greg Abbott, issued statements in support of Paxton and urged the party to come together to defeat Talarico. 

Paxton on Tuesday said "Texas will be the radical left's number-one priority," adding "this campaign is not about red versus blue, it's about so much more." 

"James Talarico is a threat to everything we hold dear in this state and in our country," Paxton said. He added later, "every Republican knows that if we lose this state, if Republicans lose this state, we lose the country." 

In a statement, Talarico thanked Cornyn for his time representing the state, and added, "To Senator Cornyn's supporters: you have a place in our campaign."

Asked last week whether the president's endorsement of Paxton would make it more difficult for Republicans to hold onto control of the Senate, Majority Leader John Thune replied that "the majority runs through a lot of different states."

It's the latest victory for a Trump-endorsed candidate taking on an incumbent who ran afoul of the president: Sen. Bill Cassidy earlier this month did not advance to a runoff in that state's primary after Mr. Trump backed a challenger, and Rep. Thomas Massie of Kentucky lost a primary last week against a Trump-backed candidate. 

Paxton was first elected Texas attorney general in 2014, aggressively taking on the Obama administration when he assumed office. He then became a close ally of Mr. Trump's, going so far as to file the lawsuit challenging former President Joe Biden's win in the 2020 election in Pennsylvania, Michigan, Georgia and Wisconsin. That case was tossed out by the U.S. Supreme Court.

Paxton also has a history of legal issues related to his role as attorney general. In 2015, he was indicted on securities fraud charges, but they were later dropped in 2024 as part of a pre-trial diversion deal. In 2023, he was impeached for bribery, dereliction of duty and disregard of official duties by the GOP-majority Texas House of Representatives in a 121-23 vote. He was later acquitted by the state Senate. Mr. Trump posted a message in support of Paxton after the trial.