Republicans Kelly Loeffler and David Perdue are neck and neck with Democrats Raphael Warnock and Jon Ossoff.

US President-elect Joe Biden's Democrats need to win both seats to gain full control of Congress.

The Republican party of outgoing President Donald Trump needs only to win one in order to retain the Senate.

All four candidates were in a dead heat with 98% of ballots counted from Georgia's 159 counties.

Mr Warnock has a wafer-thin lead over Ms Loeffler, while Mr Perdue is tied with Mr Ossoff.

Thousands of votes remain to be counted in the Atlanta suburbs such as DeKalb County, which is expected to go heavily for the Democrats. The BBC's US partner CBS News still rates both races as toss-ups.

Georgia election official Gabriel Sterling told CNN that final results were expected by lunchtime on Wednesday.

More than three million votes - about 40% of the state's registered voters - were cast before Tuesday. Early voting was a key benefit for Mr Biden in November's White House election.

Meanwhile, Mr Trump - whose unsubstantiated claims that he was the victim of electoral fraud left Republican strategists worried about turnout in Tuesday's Senate runoffs - continued to cast aspersions on the integrity of the vote in Georgia.

On Saturday, Mr Trump pushed Georgia's top election official Brad Raffensperger, a fellow Republican, to "find" enough votes to overturn Mr Biden's presidential election win in the state.

Mr Trump's unproven claims of voter fraud may have eroded voter confidence in the election system, according to exit polls from Edison Research.

Its survey of voters leaving polling stations found around 70% of them were very or somewhat confident their votes would be counted accurately, a nearly 15% drop from November's White House election.

Exit polls showed Georgians in a clean split over which party they want to control Congress: 49% favoured Republicans, while 48% said the Democratic party.

The demographics roughly matched that of November's election. Black voters made up 29% of the vote, and these voters favoured the Democratic candidates nine-to-one. The Republicans, meanwhile, were winning a majority of white voters.

And these surveys showed that most voters were repeating the choices they made in November. Georgians who supported Mr Trump were casting ballots for Mr Perdue and Ms Loeffler, while Biden supporters were doing the same for Mr Warnock and Mr Ossoff.

The vote will decide the balance of power in the Senate.

If both Democrats win, the Senate will be evenly split 50-50, allowing incoming Democratic Vice-President Kamala Harris the tie-breaking vote.

This would be crucial for pushing through Mr Biden's agenda, including on key issues such as healthcare and environmental regulations - policy areas strongly contended by Republicans.

The Senate also has the power to approve or reject Mr Biden's nominees for cabinet and judicial posts.

If Mr Ossoff and Mr Warnock both win, it would bring the White House, Senate and the House of Representatives under Democratic control for the first time since President Barack Obama's election in 2008.

None of the candidates reached the 50% needed to win outright in the elections in November, forcing Tuesday's runoff elections under Georgia's election rules.

Mr Perdue nearly won first time out against Mr Ossoff, a former filmmaker, in November, falling just short of the needed majority with 49.7%.

The other seat had more candidates, with Democrat Mr Warnock recording 32.9% to Ms Loeffler's 25.9%.

A Democrat has not won a Senate race in Georgia in 20 years but the party has been boosted by Mr Biden's presidential election win over Mr Trump there. Mr Biden's margin of victory was about 12,000 votes among five million cast.

If elected, Mr Warnock would become Georgia's first black US senator and 33-year-old Mr Ossoff, a former filmmaker, would be the Senate's youngest member.

Mr Warnock serves as the reverend of the Atlanta church where assassinated civil rights leader Martin Luther King Jr grew up and preached

Members of the BBC's voter panel in Georgia have told us what motivated them to vote.

Steven Burkhart, 53, an independent voter from Atlanta who owns a small business, says that "the idea of the Democrats controlling the government is very frightening to me".

He disagrees with the Democrats' police reform policies and says the party has a "mentality" of wealth redistribution - "and I just don't think that's very conducive to a good economy".

Robert Patillo, 36, a Democrat from Atlanta who cast his absentee ballot on the first day of voting, says that "the Democrats are running on a platform of reality".

"If you look at campaign ads, the Republican candidates are saying we need to save Western civilisation and fight back against socialism, communism and Marxism, but they never talk about real issues that impact Georgians."

"Neither of them has a plan to address the coronavirus or an economic platform that would help the average person."

It's just over two weeks until Joe Biden's inauguration, but the first real test of his presidency is on Tuesday.

If Democrats pick up the two seats and forge a 50-50 tie in the upper chamber, it's still far from certain that Biden will be able to enact the kind of sweeping legislation on the environment, healthcare and the economy that he proposed during his successful presidential campaign. The narrowness of the margin will ensure that any laws will have to be supported by centrists in his party, like Joe Manchin of West Virginia and Arizona's two senators.

It will, however, give the new president a fighting chance at legislative accomplishments - and make it significantly easier for him to appoint the administration officials and federal judges of his choice.

If the Republicans hold on, then Democratic hopes will rest on the whims of Majority Leader Mitch McConnell and a handful of Republican moderates.

On Wednesday, more political drama will unfold in Washington DC as lawmakers gather in a special joint session to ratify the results of November's presidential election. 

The typically procedural affair - which will affirm Mr Biden's victory - has become unusually contentious, with about a dozen Republican senators vowing to challenge the results.

The group, led by Senator Ted Cruz and including Ms Loeffler, wants a 10-day delay to audit unsubstantiated claims of election fraud. The move is all but certain to fail as most senators are expected to endorse the results that have already been certified by US states. 

Vice-President Mike Pence is set to preside over the session in his role as president of the Senate. 

He has come under pressure this week from Mr Trump to reject the certification, but the vice-president told Mr Trump at their weekly lunch on Tuesday that he has no power in Congress to block Mr Biden's win, according to the New York Times. Mr Trump said the report was "fake news".

Supporters of Mr Trump are demonstrating in the capital, disputing the presidential election. Mr Trump is expected to address a protest in the nation's capital on Wednesday. The mayor has asked for the National Guard to be deployed in the city amid fears of unrest.

Mr Biden, a Democrat, is due to be inaugurated as president on 20 January. 

President Trump has refused to concede the election to Mr Biden, who won 306 votes to Mr Trump's 232 in the US electoral college, which confirms the president.

Mr Biden won at least seven million more votes than the president.