Republican Texas Governor Greg Abbott will try to hold off challengers on his right flank on Tuesday to set up a likely November election against Democratic former congressman Beto O'Rourke as the state holds the first nominating contest of the US midterms.
Polls are open until 7pm local time on Tuesday, with early voting figures released by the Texas Secretary of State's office showing that over a million Republicans had voted early, compared with about 627,000 Democrats.
Jointly, that's less than 10 per cent of registered voters, a sharp drop from the 2020 elections, during which there was a 57 per cent turnout in early voting. Experts interpret early voting as a sign of enthusiasm.
Voters will also pick their parties' nominees for the US House of Representatives and other statewide offices in primary elections that will provide the first test of a wave of new restrictions on voting passed in response to former president Donald Trump's false claims of election fraud.
The results will offer clues about the mood of American voters who decide on November 8 which party will control Congress for the rest of President Joe Biden's four-year term in office.
The voting comes the same day that Mr Biden delivers his annual State of the Union address.
Midterms typically serve as a referendum on the sitting president and Republicans are favoured to win a majority in at least one of the two chambers of Congress that Democrats control by razor-thin margins.
Doing so would allow them to block Mr Biden's legislative agenda and to launch potentially damaging investigations into his administration.
Texas's Republican attorney general, Ken Paxton, is running for a third term, despite his pending 2015 indictment for securities fraud. Mr Paxton has denied any wrongdoing and has been endorsed by Mr Trump.
His Republican opponents include firebrand US Representative Louie Gohmert, Land Commissioner George P Bush, the grandson of former president George H W Bush, and former state Supreme Court justice Eva Guzman.
Under Texas law, if no candidate exceeds 50 per cent of the vote, the two candidates who receive the most votes advance to a run-off election on May 24.
Republican politicians and Mr Abbott muscled sweeping new voting restrictions through the state legislature last year over objections from Democrats and civil rights groups. In the past few weeks, elections officials in several counties have reported an unprecedented number of rejected mail-in ballots due to new identification requirements.
Mr Abbott's most notable opponents, former Texas Republican Party chairman Allen West and former state senator Don Huffines, lag far behind; polls suggest Mr Abbott is likely to avoid a run-off altogether.
The governor secured Mr Trump's endorsement last summer and has nearly $50 million in campaign funding.