In duelling rallies, President Donald Trump and President-elect Joe Biden have implored Georgia voters to turn out for elections on Tuesday that will decide which party controls the Senate.
Mr Trump, a Republican, and Mr Biden, a Democrat, said the vote would shape America for years to come.
More than three million Georgians have already cast ballots – nearly 40% of the state’s registered voters.
If the Democrats win, they will control all of Congress and the White House.
With the Senate on a knife-edge, Asian shares plunged early on Tuesday amid the political uncertainty in the world’s biggest economy.
The Republican Senate incumbents in Georgia, Kelly Loeffler and David Perdue, are fighting for their political lives against two Democratic challengers, Jon Ossoff and Reverend Raphael Warnock.
“Georgia, the whole nation is looking to you,” Mr Biden said at a drive-in rally in Atlanta on Monday, emphasising that control of the Senate would mould prospects for his incoming administration.
“Unlike any time in my career, one state – one state – can chart the course, not just for the four years but for the next generation,” he said, flanked by Mr Ossoff and Mr Warnock.
The Senate has the power to approve or reject Mr Biden’s nominees for cabinet and judicial posts, as well as the bulk of his policy wish-list.
On Monday evening, President Trump told voters in Dalton, Georgia, that the Georgia runoff was the “last line of defence” against the Democrats.
In what may have been the last rally of his presidency, he said: “If the liberal Democrats take the Senate and the White House – and they’re not taking this White House – we’re going to fight like hell, I’ll tell you right now.”
But the president also repeated unproven allegations that he was only declared the loser in Georgia after November’s White House election because of fraud, claims that Republican officials are worried could now depress turnout among the party faithful in Tuesday’s vote.
Mr Trump – who is due to leave office on 20 January – also hinted that he wanted Vice-President Mike Pence, in his role as president of the Senate, to reject Mr Biden’s win when Congress meets on Wednesday to certify the election results.
“I hope that Mike Pence comes through for us,” Mr Trump said. “Of course, if he doesn’t come through, I won’t like him quite as much.”
Nearly 3 million Georgia voters – around half of those that voted in the November general election – have already cast their ballots.
The eve-of-poll rallies came on the heels of a controversial phone call between the president and Georgia’s top election official, Secretary of State Brad Raffensperger.
In a recording of the call, first published by the Washington Post newspaper on Sunday, Mr Trump pressured Mr Raffensperger to “find” votes that would reverse his defeat in the state.
Mr Biden won 306 votes to Mr Trump’s 232 in the US electoral college, which confirms the US president. Mr Biden also won at least 7 million more Americans’ votes than the president.
At his rally on Monday, Mr Biden did not make direct reference to the call, but alluded to Mr Trump’s persistent challenges to the election results, saying that “politicians cannot assert, take or seize power”.