President Donald Trump has endorsed Texas Attorney General Ken Paxton over longtime GOP Sen. John Cornyn in Tuesday’s high-stakes Republican Senate runoff.

Trump called Paxton a “true MAGA Warrior” and criticized Cornyn as “very disloyal,” citing his past lack of support during tough times.

The winner will face Democrat James Talarico in November for a seat that could help determine control of the narrowly divided U.S. Senate.

The race has become a major test of Trump’s influence within the GOP. Cornyn has warned that nominating Paxton would trigger a massive wave of Democratic spending and put the seat at risk.