Democrats will take control of the US Senate if they win runoff elections in Georgia on Tuesday as the two contests will put much of Joe Biden's agenda as president on the table.

President Donald Trump and President-elect Biden will be in Georgia for campaign rallies on Monday, repeat appearances for both the outgoing and incoming presidents.

Biden will be in Atlanta for Jon Ossoff and Raphael Warnock. Trump plans to hold a final rally on the same day for Republican Senators Kelly Loeffler and David Perdue in Dalton, in northwest Georgia where the early voting turnout has trailed other parts of the state.

Perdue isn't doing any last-minute campaigning because he is quarantining after an unidentified individual with whom the senator had been "in close contact" tested positive for the coronavirus, meaning he'll likely skip Trump's rally Monday.

No candidate in either of Georgia's Senate races won a majority of the vote on Nov 3, triggering the runoff for both seats.

"Something like this has never happened before and probably never will again," University of Georgia political scientist Charles Bullock told Bloomberg News. "Two seats, from one state, in one election, that will decide Senate control. It's just unprecedented."

The runoffs are so important to both parties that about $800 million has already poured into the contests, most of it financing TV and radio ads.

If both Democrat candidates win, the Senate will be split 50-50 between the parties. Vice President-elect Kamala Harris would then cast any tie-breaking vote, effectively flipping control of the chamber to Democrats.

But a victory by either Republican would give Senate Majority Leader Mitch McConnell 51 votes, a slim majority but enough to curtail Biden's initiatives and hold the power to confirm or reject picks for his Cabinet and the federal judiciary,

Democrats are looking to repeat Biden's victory in Georgia in the Nov 3 general election. He beat Trump by 12,670 votes, according to the audit required by state law, and became the first Democratic presidential candidate to carry the state since 1992.

In an hourlong phone call on Saturday released by The Washington Post on Sunday, Trump told Georgia Secretary of State Brad Raffensperger that there was "no way'' he lost the state and suggested to the fellow Republican that he find enough votes to overturn his defeat in the state.

"The people of Georgia are angry; the people in the country are angry," Trump said, according to one audio excerpt published online by the Post. "And there's nothing wrong with saying, you know, um, that you've recalculated."

During the telephone call, the Post said Trump alternately flattered, begged and threatened Raffensperger with vague criminal consequences in an attempt to undo his loss.

Raffensperger and his office's general counsel rejected Trump's assertions throughout, according to the audio excerpts and the newspaper's account.

Trump on Sunday in a tweet confirmed he had spoken to Raffensperger.

On Friday evening, Trump asserted on Twitter that the runoff races are "illegal and invalid".

Some Republican leaders are concerned that his supporters will take the president's argument seriously and decide not to vote, which could hand the election to the Democrats.

More than 70,000 new voters have registered in Georgia since the presidential election on Nov 3.

According to Georgia Votes, which is analyzing data from the secretary of state's office, 3,001,017 voters had cast ballots in the runoff elections following the last day of the three-week advance in-person voting period.

Of those votes, 928,069 are absentee by mail and 2,072,948 are from in-person early voting. Hundreds of thousands more ballots are likely to be cast on Tuesday.

No votes will be counted until polls close at 7 pm on Tuesday, and the results may not be known for days. The counts will almost certainly face legal challenges.

State election data analyzed by The Atlanta Journal-Constitution indicate more ballots have been cast so far in areas that tend to favor Democrats. Black voters, who generally support Democrats, made up a higher portion of voters so far than in the presidential election, the newspaper said.

But Republicans could make up ground with a strong showing on Election Day, as in the general election.