Democratic US Senator Raphael Warnock has beaten Republican former football star Herschel Walker a tight race in Wednesday’s runoff election in Georgia.

The result means the Democrats will expand their razor-thin US Senate majority to 51-49.

With 72 per cent of the estimated vote tallied on Wednesday afternoon (AEDT), Reverend Warnock led Mr Walker by 50.4 per cent to 49.6 per cent, according to Edison Research.

Final results were not expected until later in the night. However, by mid-afternoon on Wednesday, multiple US media outlets, including CNN, CBS and The Associated Press, had called the race for Reverend Warnock.

Wednesday’s election rerun was a last test of Donald Trump’s clout with midterm voters as he seeks the Republican nomination to challenge US President Joe Biden in 2024. The former president, who endorsed Mr Walker, had a mixed record in his most competitive endorsements for Congress in this year’s elections.

Within minutes of the Georgia result being called, recriminations began.

CNN reported that prominent Iowa Republican Bob Vander Plaats, a one-time ally of Mr Trump – tweeted the loss would be “another blow to former President Trump.

“Conservatives across the country are tired of losing. 2024 is key to winning the future again: Choose well,” he added.

Reverend Warnock held a commanding lead in the state’s biggest counties, including the Democratic stronghold of Atlanta. But Mr Walker led in Republican-heavy rural counties, where vote tallies were slower to be reported.

Mr Walker’s campaign was plagued by repeated gaffes and a variety of allegations, including claims by former girlfriends that he paid for their abortions, even though he has campaigned for the procedure to be outlawed. He has denied the accusations.

The race was the most expensive of the 2022 US midterm election season, with more than $US400 million ($597 million) spent. The contest went to a runoff after neither candidate secured 50 per cent of the vote on November 8.

A victory by Reverend 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 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 Reverend 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 in Georgia cast their votes before election day, equal to 47 per cent of the November 8 turnout. Analysts say the early votes likely tilted Democratic, which meant Mr Walker needed 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,” Reverend Warnock told a crowd in Norcross, Georgia, on Tuesday.

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

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. Reverend Warnock edged him 49.44 per cent to 48.49 per cent in November, even as Republican Governor Brian Kemp and other statewide Republican candidates easily won re-election.

At a diner in Marietta, Mr Walker deflected questions about his character, which Reverend Warnock made a centrepiece of his campaign.

“Right now I put my character up against Raphael Warnock any day,” Mr Walker said on Tuesday.

This is the third Senate runoff in two years for Georgia.

-with AAP