Texas voters will decide Tuesday between incumbent Sen. John Cornyn and state Attorney General Ken Paxton in a Republican primary runoff that will determine the GOP nominee for the U.S. Senate seat.

The two candidates have kept a low public profile on the final day of campaigning Monday, though both have maintained television advertising campaigns leading up to the election. Neither candidate has scheduled major public events as the campaign concludes.

Cornyn, who has served in the Senate since 2002, advanced to the runoff after failing to secure a majority in the initial primary election. Paxton, who has served as Texas Attorney General since 2015, emerged as his primary challenger in a field that required a second round of voting.

The runoff became necessary when no candidate received more than 50 percent of the vote in the initial Republican primary. Texas law requires a majority winner in primary elections, triggering automatic runoffs when no candidate reaches that threshold.

The winner of Tuesday's runoff will face the Democratic nominee in the November general election. Texas has not elected a Democrat to the U.S. Senate since 1988, making the Republican primary the decisive contest for the seat.

Polling locations across Texas will be open Tuesday for the runoff election, with results expected to be reported throughout the evening as votes are counted.