Polls have closed in a hard-fought runoff election between Democratic Senator Raphael Warnock and Republican former football star Herschel Walker that will determine whether Democrats can expand their razor-thin US Senate majority.

The race is a last test of Donald Trump’s clout with voters as he seeks the Republican nomination to challenge President Joe Biden in 2024.

The former president had a mixed record in his most competitive endorsements for Congress in the November midterm elections, including Mr Walker.

Mr Walker’s campaign has been plagued by repeated policy gaffes and a variety of allegations, including claims by former girlfriends that he encouraged them to get abortions, although he has campaigned for the procedure to be outlawed. He has denied the accusations.

The runoff set early voting records in Georgia in a race that has become the most expensive of the 2022 US midterm election season, with more than $400 million ($590 million) spent so far. The contest went to a runoff after neither candidate secured 50 per cent of the vote on November 8.

A victory by Mr Warnock would give Democrats a 51-seat majority in the 100-seat Senate, which would make it slightly easier to advance Mr Biden’s nominees for judicial and administrative posts. Most legislation would still require Republican support.

A Mr Walker win, on the other hand, would keep the Senate at its current 50-50 split, with Democrats controlling the chamber thanks to Vice President Kamala Harris’s tie-breaking vote.

On a wider scale, a Mr Warnock victory could solidify Georgia as a battleground in presidential elections. It also would be another midterm defeat at the polls for a protege of Mr Trump, who spurred Mr Walker to run.

At least 1.9 million people cast their votes before Election Day, equal to 47 per cent of the November 8 turnout, and state election official Gabe Sterling has said he expected one million more would vote on Tuesday.

Analysts say the early votes likely tilted Democratic, which means Mr Walker will need strong Election Day turnout from his supporters to overcome the gap.

“We should not rest on our laurels. The job is not done. The truth is my opponent could still win this election,” Mr Warnock told a crowd in Norcross, Georgia, on Tuesday.

Mr Warnock is pastor of the historic Atlanta church where assassinated civil rights leader Reverend Martin Luther King Jr preached.

Along with the abortion claims, former girlfriends have accused Mr Walker of domestic abuse. He has also faced accusations that he maintains his primary residence in Texas, not Georgia.

Mr Walker has denied the charges, but they may have discouraged some Republican voters. Mr Warnock edged him 49.44 to 48.49 per cent in November, even as Republican Governor Brian Kemp and other statewide Republican candidates easily won re-election.

This is the third Senate runoff in two years in the state and the second for Mr Warnock, who first won the seat in a runoff in January 2021.

Republicans won a narrow majority in the US House of Representatives in the November 8 election, but fell short of the “red wave” that some in the party had forecast, despite flagging approval ratings for Mr Biden.

-AAP