President-elect Joe Biden and outgoing President Donald Trump held competing rallies on Monday in Georgia, where voters are deciding how much leverage the Democrat will hold when he enters the White House on January 20.

The traditionally conservative state, which is undergoing significant demographic changes, is the ultimate battleground: holding the power to tip the scales in the Senate and fundamentally change politics in Washington for at least the next two years.

"Georgia, the whole nation is looking to you," Biden said at a rally in Atlanta a day before polls closed. "Unlike any time in my career, one state can chart the course not just for the next four years but for the next generation."

Wins for Democrats Jon Ossoff and Raphael Warnock on Tuesday will give the party control of the Senate and strengthen Biden's hand, from appointments to his legislative agenda. A victory for either Republican incumbent, David Perdue or Kelly Loeffler, will allow six-year Senate Majority Leader Mitch McConnell to continue calling the shots in the chamber.

Georgia has also become the central state in Trump's claims of a "rigged" presidential election – at the weekend he was recorded trying to persuade Georgia's Republican secretary of state to "find" the votes to overturn Biden's win on November 5, a theme he returned to on Monday.

"If the liberal Democrats take the Senate and the White House – and they're not taking this White House," Trump said during a rally in Dalton, Georgia on Monday. "We're going to fight like hell."

The outgoing president's attacks on the voting system risk complicating the argument Republicans want him to make: get out and vote. His insistence that he won the election is also splitting the party ahead of the Senate's certification of the presidential election results on Wednesday and could turn off moderates.

Why Georgia matters

Voting in Georgia's two runoff Senate races, held because no candidate won 50 percent of the vote on November 5, began in mid-December but reaches a climax on Tuesday.

The races, which pit 33-year-old Ossoff against 70-year-old Perdue and 51-year-old Warnock and against 50-year-old Loeffler, have broken fundraising records for the Democrats, an indication of the high stakes.

Nearly half a billion dollars has been spent on advertising, according to Ad Impact, while OpenSecrets calculates that Ossoff has raised $138 million, dwarfing Perdue's $89 million, while Warnock has taken in $124 million to Loeffler's $92 million.

With Democrats sitting on 48 Senate seats to the Republicans' 50, wins for Warnock and Ossoff alongside the casting vote of Vice President-elect Kamala Harris would give the party control of the chamber.

The tight margins coupled with centrist instincts of some Senate Democrats mean there would still be serious limits on Biden's plans for expanding healthcare and environmental protections, but it would all but guarantee his cabinet and judicial nominees are confirmed.

If Loeffler or Perdue, two of the wealthiest members of the Senate, prevail, McConnell will be able to disrupt and frustrate, as he did in the Obama second term. 

The Kentucky veteran, alongside some moderate Republican senators, will hold leverage over Biden's appointments and legislative agenda – likely leading to a flurry of executive orders from the new president.

Who's going to win?

The big question in Tuesday's elections, as always, relates to turnout. In runoff elections and polls in non-presidential years, the number of voters typically drops off.

Early voting returns appear to favor the Democrats, but the arithmetic from the first round of elections suggest the Republicans could have their noses in front, and on-the-day voting is again expected to benefit the incumbents.

Perdue came within a whisker of reelection on November 5, falling short of the 50 percent threshold by just 0.3 points and beating Ossoff by 1.8, less than 88,000 votes.

The race for the other seat was more complicated on November 5 because it was an open contest: over two million of the nearly five million votes cast went to candidates other than Warnock or Loeffler, as multiple candidates from the same parties ran.

The Democrat came out on top with 32.9 percent, but it has little relevance to the runoff in which he will go head to head with Loeffler, who was appointed to her Senate seat in 2019.

The polls suggest both races will be nail-biters: a Fox 5 survey published on Monday put all four candidates on 49 percent; a Trafalgar Group poll on December 29 gave the Democrats narrow advantages, but the same organization put the Republican pair ahead just a week earlier.

Early voting numbers indicate the Democrats have made progress — more than three million ballots have already been cast, according to the U.S. Elections Project, including 118,000 people who did not vote in the 2020 general election with turnout particularly high in Democratic strongholds around the city of Atlanta.

Just as in the general election, the Republican candidates are expected to have a lead on election night, which narrows as the early votes are counted.

And just as in November, the outcome may not be known for some time. If the final results are within half a percentage point, the losing candidate has the right to request a recount.