Early voting in Texas started Monday, as Trump tries to stir up GOP enthusiasm and prevent a stunning upset. Despite his staggering cash haul and quick rise to national prominence, O'Rourke still appears to be an underdog. No recent independent poll has shown him closer than 5 percentage points behind Cruz.

O'Rourke rallied with supporters in Houston on Monday ahead of the president's rally for Cruz, who was one of the several Republicans who lost to Trump in the 2016 GOP primary. O'Rourke's campaign did not immediately respond to a request to comment on the president's visit.

An O'Rourke victory would alter the battle for control of the Senate, giving Democrats a much better chance of gaining control of the chamber than they otherwise would have. The GOP looks to keep or expand its 51-49 majority in the chamber.