Voters in the US state of Georgia have cast their ballots in the final Senate contest up for grabs after last month’s midterm elections.
The close race pits Democrat Raphael Warnock, a pastor, against Republican Herschel Walker, a football legend.
Because no candidate passed 50% of the vote in November, the seat had to be decided by a run-off.
The result will determine whether Democrats win an outright majority in the lower chamber of the US Congress.
The bitterly fought contest will also test former President Donald Trump’s influence after candidates he endorsed in November’s midterms saw mixed results.
Counting is underway after polls closed at 19:00 local time on Tuesday (midnight GMT), and no US television network has so far projected a result.
On election eve, most opinion polls showed Mr Warnock – a southern Baptist preacher – in a tight race with Trump-backed Mr Walker.
Mr Warnock – who won by about 37,000 votes in November’s vote, not enough to cross the 50% threshold for victory – said on Tuesday he hoped rain would not dampen turnout.
“I hope that people will continue to show up – I’ve seen people stand in the rain for a concert and there’s really too much at stake,” the 53-year-old said.
Mr Walker’s campaign has been dogged by claims – which he denies – that he paid for two former girlfriends’ abortions, despite his calls for the procedure to be outlawed.
The political newcomer also had to acknowledge during the campaign that he had fathered three children out of wedlock, despite having long railed against absentee fathers.
The 60-year-old Republican predicted victory at his sole campaign stop on Tuesday at a Marietta diner.
“We’re going to win by 100,000,” he said.
Election day voting numbers in Georgia were expected to have reached one million by the close of the polls, said election official Gabriel Sterling.
A record-breaking 1.9 million Georgians had already cast early or postal ballots, including a daily record of 352,953 on the last day of early voting on Friday alone.
Mr Warnock’s campaign has spent about $170m, while Mr Walker’s could only muster nearly $60m, according to federal filings.
Mr Walker’s Senate bid is the last Republican opportunity to flip a Senate seat following midterm flops by Trump-backed Senate candidates in New Hampshire, Nevada, Arizona and Pennsylvania. Other candidates he championed won in Ohio, Wisconsin and North Carolina.
Both US President Joe Biden, who has had low approval ratings, and Mr Trump largely avoided wading into the race.
Following the midterms, Republicans have won a slender majority in the US House of Representatives.
Mr Biden’s Democrats have managed to maintain their razor-thin control of a 50-50 Senate, thanks to Vice-President Kamala Harris’ tiebreaking vote.
If Mr Warnock, the incumbent, can win re-election on Tuesday, the Democrats would run the chamber by 51-49.
This would mean that most legislation would still need Republican support, but it would be slightly easier for Mr Biden to appoint judges and members of his administration.