Democrats and Republicans are neck-and-neck in two critical US Senate races as votes are counted in Georgia contests that will decide whether resident-elect Joe Biden enjoys control of Congress.

The leads swung back and forth on Tuesday, with Republican incumbents David Perdue and Kelly Loeffler holding slight edges on Democratic challengers Jon Ossoff, a documentary filmmaker, and Reverend Raphael Warnock, a pastor at a historic black church in Atlanta.

With about 78 per cent of the expected vote in, the Republicans were ahead with Ms Loeffler leading Mr Warnock by a percentage point and Mr Perdue leading Mr Ossoff by 1.6 percentage points, according to Edison Research.

But in a hopeful sign for Democrats, about 670,000 votes remained to be counted in counties Mr Biden won in November, mostly around Atlanta, with about 300,000 left to count in counties President Donald Trump carried, according to Edison estimates.

An Edison exit poll of more than 5200 voters found half had voted for Mr Trump in November and half for Mr Biden.

The voters were also evenly split on whether Democrats or Republicans should control the Senate.

The survey included both early voters and voters who cast ballots on Tuesday (local time).

Democrats must win both contests in Georgia to take control of the Senate.

A double Democratic win would create a 50-50 split in the Senate and give vice president-elect Kamala Harris the tie-breaking vote after she and Mr Biden take office on January 20.

The party already has a narrow majority in the House of Representatives.

If Republicans hold even one of the two seats, they would effectively wield veto power over Mr Biden’s political and judicial appointees as well as many of his legislative initiatives in areas like economic relief, climate change, healthcare and criminal justice.

Mr Warnock and Mr Ossoff were each running about 0.5 percentage points ahead of Mr Biden’s November performance in the 103 counties where at least 95 per cent of the vote had been counted.

Tweet from @realDonaldTrump

No Democrat has won a US Senate race in Georgia in 20 years but opinion surveys show both races as exceedingly close.

The head-to-head run-off elections, a quirk of state law, became necessary when no candidate in either race exceeded 50 per cent of the vote in November.

Mr Biden’s narrow statewide win in the November 3 election – the first for a Democrat since 1992 – has given the party reason for optimism in a state dominated by Republicans for decades.

Tweet from @JoeBiden

More than three million Georgians voted early by mail or in person, shattering the record for run-off elections even before election day arrived.

The two races drew nearly half a billion dollars in advertising spending since November 3, a staggering total that fuelled a tsunami of television commercials.

In Smyrna, about 26 km northwest of Atlanta, Terry Deuel said he voted Republican to ensure a check on Democratic power.

“The Democrats are going to raise taxes,” the 58-year-old handyman said.

“And Biden wants to give everyone free money – $US2000 ($A2600) each or something like that for COVID stimulus? Where are we going to get the money?”

The campaign’s final days were overshadowed by Trump continuing to subvert the presidential election results.

On Saturday, he pressured Georgia Secretary of State Brad Raffensperger, a fellow Republican, on a phone call to “find” enough votes to reverse Mr Biden’s victory.

Mr Trump’s efforts to undo his loss – with some Republicans planning to object to the certification of Mr Biden’s win when Congress meets on Wednesday to count the presidential vote – have caused a Republican party split.

-AAP