Voters headed to the polls for runoff elections in the southern state of Georgia on Tuesday that will determine control of the US Senate for the first years of Joe Biden's presidency.

The 78-year-old Democrat, who takes office on January 20, and outgoing president Donald Trump both visited the Peach State on Monday to make last-ditch appeals for their candidates.

Polls opened at 7:00 am (12:00 GMT) and start closing at 7:00 pm (00:00 GMT). A record 3 million-plus people have voted early amid the coronavirus pandemic, election officials say, and final results may not be known for several days.

"It's the most important election in my lifetime," said Robert Lowe, a 74-year-old retired improv comedian, after casting his ballot for the Democrats on the ticket, Jon Ossoff and the Reverend Raphael Warnock.

If Republicans retain control of the Senate there will be "continued obstruction" in Washington, Lowe told AFP after casting his ballot at the Lutheran Church of the Redeemer in Atlanta.

Georgia has been reliably Republican but Biden beat Trump in the state by nearly 12,000 votes in the November 3 presidential election and polls have the pair of Senate races neck-and-neck.

Republicans hold 50 seats in the Senate and a victory in just one of the runoff races would give them a majority and the ability to thwart Biden's agenda.

A Democratic sweep would result in a 50-50 split with Democrats holding the tie-breaking vote in Vice President Kamala Harris.

Harris has also been campaigning for Ossoff, a 33-year-old television producer, and Warnock, the 51-year-old African-American pastor at the Atlanta church where Martin Luther King Jr once preached.

"Your vote is your voice, and your voice is your power," Harris tweeted on Tuesday morning. "Now is the time to stand up, speak out, and vote for a better future."

‘Last line of defence’ 

The Republican and Democratic parties have made Georgia their political ground zero, with thousands of volunteers leaving no door unknocked and the candidates barnstorming the state.

Ossoff is running against David Perdue, a 71-year-old business executive who was elected to the Senate in 2014.

Warnock is facing off against Kelly Loeffler, 50, a businesswoman who was appointed to the Senate in December 2019 after the serving senator resigned for health reasons.

The pair of Senate races are the most expensive congressional runoffs in US history — according to the Center for Responsive Politics a staggering $832 million has been spent including on the primaries and general election, where no candidate cleared the 50 per cent threshold required for victory.

If Democrats flip both seats they would effectively hand Biden all the levers of political power in Washington and enable him to enact his ambitious legislative agenda.

"One state — one state! — can chart the course, not just for the next four years but for the next generation,” Biden told a rally in Atlanta on Monday.

Republicans argue that keeping Senate control would serve as a check on the incoming Biden administration.

"The stakes in this election could not be higher,” the 74-year-old Trump told a rally in Dalton, Georgia, where he appeared on stage with Loeffler. "Make sure your vote is counted.”

Tuesday’s runoffs are "your last chance to save the America that we love”, he said.

Perdue, the other Republican candidate, is in quarantine after coming into contact with someone who tested positive for COVID-19.

‘Find 11,780 votes’ 

Trump spent most of the rally rehashing his baseless claims that he won the presidential election.

The event came a day after The Washington Post published a recording of a phone call in which the president attempted to pressure Georgia officials to reverse the certified vote and hand him victory in the state.

On the tape, Trump is heard telling Georgia Secretary of State Brad Raffensperger he wants to "find 11,780 votes” — one more than Biden’s margin of victory in the state.

Democrats are counting on a large mobilisation of Black voters and hoping some Republicans may be discouraged from voting due to Trump’s relentless accusations of fraud.

The runoffs are taking place ahead of a joint session of Congress on Wednesday to certify the Electoral College vote that confirmed Biden as the presidential winner.

Certification is usually a formality but dozens of House Republicans and 12 Senate Republicans loyal to Trump — including Loeffler — have said they will raise objections.

Such a move would prompt a debate and a vote that is doomed to fail in the Democratic-controlled House and in the Senate, where leading Republicans have acknowledged Biden’s victory.

Large street protests in support of Trump are planned in Washington on Wednesday, prompting a heavy police presence and the deployment of the National Guard amid fears of clashes.