In dueling rallies, President Donald Trump and President-elect Joe Biden have urged voters in Georgia 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 votes for two Senate seats would shape the US for years.
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.
The Republican Senate incumbents in Georgia – Kelly Loeffler and David Perdue – are trying to hold off the Democratic challengers, Jon Ossoff and Reverend Raphael Warnock.
The runoff is needed under Georgia’s state election rules because none of the Senate candidates received more than 50% of the vote in November’s US election. Voting begins at 07:00 (12:00 GMT) and ends about 12 hours later.
What’s at stake in Georgia?
The vote will decide the balance of power in the US Senate.
Republicans are currently in control, holding 52 of the 100 seats. If both Democrats win on Tuesday, the Senate will be evenly split, allowing incoming Democratic vice-president Kamala Harris the tie-breaking vote.
This would be crucial for pushing through Mr Biden’s progressive agenda, including key issues such as health care and environmental regulations – issues with strong Republican opposition.
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 will 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.
What did Biden and Trump say?
At a drive-in rally in Atlanta on Monday, Mr Biden told voters: “Georgia, the whole nation is looking to you. The power is literally in your hands.”
Flanked by Mr Ossoff and Mr Warnock, he said: “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.”
Mr Biden also took aim at Mr Trump, accusing him of “whining and complaining” about November’s presidential election result rather than concentrating on the Covid-19 pandemic.
“I don’t know why he still wants the job, he doesn’t want to do the work,” he said.
On Monday evening at a rally in Dalton, President Trump told voters 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 told the state’s voters “the whole world is watching” and that this was “your last chance to save the America that we love”.
The president repeated unproven allegations that he was only declared the loser in Georgia after November’s White House election because of fraud.
Republican officials are worried this could depress turnout among the party faithful in Tuesday’s vote. Mr Trump played this down, telling voters to “swarm it tomorrow”.
A Democrat has not won a Senate race in Georgia in 20 years, but the party will be buoyed by Mr Biden’s presidential election win over Mr Trump there. The margin of victory was about 12,000 votes among five million cast.
Mr Biden won 306 votes to Mr Trump’s 232 in the US electoral college, which confirms the US president. Mr Biden won at least seven million more votes than the president.