What happens if there is a 50-members tie in the Senate by Republicans and Democrats?
The Senate can provide a useful function of checks and balances in Congress, or it can stubbornly prevent fruitful actions being taken. Georgia could be key.
Today is the day! Millions have already voted, and now the remainder of those registered to vote in Georgia go to the polls to make their voices heard. And it's not only the future of the southeastern US state that is at stake, the agenda of President-elect Joe Biden also hinges on the two Senate runoff races.
Georgia deciding Senate future and more
The high-stakes campaign that has unfolded since 3 November when Biden defeated President Donald Trump in the presidential election, has obliterated spending records and spurred unprecedented turnout. Political groups have flooded the southern state with a tsunami of television advertising. Biden and Trump both made their voices heard on Monday at rallies in the state, underscoring the political stakes of the contests.
If either or both Republican incumbent senators - David Perdue and Kelly Loeffler - win on Tuesday, their party would retain a narrow majority, effectively giving Senate Republicans the ability to block Biden's most ambitious goals. A Democratic sweep would produce a 50-50 split, with Vice President-elect Kamala Harris holding the tiebreaker that determines control.
Democrat Jon Ossoff, a documentary filmmaker, is challenging Perdue, while the Rev. Raphael Warnock, senior pastor at the historic Black church Ebenezer Baptist Church in Atlanta, will take on Loeffler. Public opinion polls show Republican incumbents Kelly Loeffler and David Perdue have about the same level of support as their Democratic challengers.
Following are key facts on the vote-counting process and what to expect as election returns start to come in Tuesday night.
It may take a while to know the result
Slow counting of Georgia's mail ballots in November's presidential election kept the world in the dark about who won the state until three days after polls closed. Biden ended up winning the state by less than 12,000 votes out of about five million cast.
A similar delay could unfold after Tuesday's contests if the races are really close, said Walter Jones, a spokesman for the office of Georgia's top election official, Secretary of State Brad Raffensperger.
"We may be looking at several days," Jones said, adding that the delay would mostly come from mail ballots returned on Election Day.
Election officials cannot start counting ballots until 7 p.m. on Tuesday, though they have started processing them, such as checking that voter signatures on mail ballots match those on record.
Around one million mail-in ballots have been processed, state data shows. These ballots, as well as more than two million cast in person at early voting centres, will likely be counted quickly on Election Night.
But for the mail votes that arrive on Election Day, officials will still need to open envelopes, check signatures and load the ballots in counting machines. They will also need to process hundreds of thousands of votes expected to be cast in person that day.
Another potential delayed blue wave
In November, mail ballots heavily favoured Biden in Georgia and other swing states, while President Donald Trump led in votes cast in person. If more Democrats again vote by mail this time, initial results could similarly show Republicans in a lead that gives way to Democrats catching up.
The slow counting of mail ballots was a key reason why Trump took an early lead on Election Night on 3 November, only to trail Biden in the days that followed as more mail ballots were counted.
In the end, about a quarter of Georgia's ballots in November were cast by mail and Biden won about two-thirds of them, state data shows. The rest were cast in person, with about 55% going to Trump.
"Initial results will generally reflect the state of the race but probably will be a little redder than what they will end up being," said Kyle Kondik, a political analyst at the University of Virginia's Center for Politics.
A demographic change since November
People who were too young to vote on 3 November but who have since turned 18 will now get their chance to have their say on who occupies Georgia’s Senate seats.
In an interview with CNBC back in July last year, Ossoff said he believed youngsters would be key to helping him take Perdue's seat, saying they “get” his policy priorities, such as a drive for more sustainable energy sources and support for expanding healthcare coverage.
“I think that young people have a very clear-eyed understanding of what’s happening in our country and in our world and where we need to get to," he said.
What about a recount?
Georgia did two recounts of November's presidential contest: a hand recount of paper ballots ordered by the state's top election official, and a subsequent recount using computer scanners that was requested by the Trump campaign.
Georgia allows a losing candidate to force a recount if the margin of victory is less than or equal to 0.5% of the total vote in the race. A recount must be requested within two days of the results being certified by election officials.
Candidates can also request a recount if they think there has been an error in the tabulation; in that case it's up to the secretary of state to decide whether to conduct one. Local election officials also have the power to recount results in their county before the results are certified.
Live coverage of the US presidential election
You can stay up-to-date with the latest developments in the US run-off elections with our dedicated rolling feed. We'll be keeping you abreast of everything related to what's going on In Georgia until a final decision is confirmed.