Texas Attorney General Ken Paxton ousted four-term U.S. Senator John Cornyn on Tuesday, riding President Donald Trump's endorsement to a runoff victory for the Republican Senate nomination, media projected.

The race was called shortly after polls closed in Texas' westernmost counties.

"Thank you, Texas. Together, we just made history," Paxton wrote on X. "Now, we must unite to defeat the most well-funded, radical Democrat in America."

Paxton's victory will force Cornyn into retirement early next year - and the Republican political establishment in Washington into embracing a candidate it has long opposed.

Cornyn, 74, was backed by Republican leadership in the fight of his political life against Paxton, a 63-year-old scandal-plagued attorney general who won Trump's endorsement last week.

Paxton will face Democratic state Representative James Talarico in a high-profile race that could help decide control of the Senate and may become one of the most expensive in U.S. history.

The 37-year-old Democrat is a Presbyterian seminarian and leading fundraiser whose campaign has appealed to independent and moderate voters.

Senate Republicans' campaign arm warned in an internal memo last year that a Paxton nomination "would hand Democrats an opening to flip Texas and cause Republicans to divert hundreds of millions of dollars that would otherwise be spent winning key battlegrounds."

Cornyn told supporters he will back the Republican ticket in November.

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

Republicans currently hold a 53-47 majority in the Senate over Democrats.