The US state of Georgia is going to the polls in a vote that will decide whether President-elect Joe Biden’s Democrats control the Senate.
Victory in the two runoff elections would give Mr Biden control over the whole of Congress and with it the power to push forward his progressive agenda.
Mr Biden said Georgians could shape the US for years to come.
Republican President Donald Trump told voters it was their “last chance to save the America that we love”.
Republicans Kelly Loeffler and David Perdue currently hold the two Senate seats in the state. Ms Loeffler is taking on Reverend Raphael Warnock and Mr Perdue is battling Jon Ossoff.
None of the candidates reached the 50% needed to win outright in the elections in November, forcing Tuesday’s runoffs under Georgia’s election rules. Voting began at 07:00 (12:00 GMT).
The vote will decide the balance of power in the US Senate.
The Republicans currently hold 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 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.
Voting should last about 12 hours, ending at 19:00 local time (midnight GMT), although all those still in line to vote will be allowed to do so. A state primary last June did not stop accepting ballots until past midnight.
Democrats are hoping for a large turnout and have been buoyed by the fact that more than three million Georgians have already cast their ballots – nearly 40% of the state’s registered voters. Early voting was a key benefit for Joe Biden in the presidential election.
The Democrats will be looking to turn out supporters in major urban areas, particularly the suburbs of Atlanta. The issue of long lines of voters could be more of a problem for them.
For the Republicans, getting out voters on the day is even more crucial, and they will be looking to the stronghold of north Georgia, as well as rural areas and smaller towns.
When will we know the result? You’d be brave to give a time. There were two recounts before Joe Biden was declared the winner over Donald Trump in November’s presidential poll. Generally, results come in quickly but if these races are close, it could be days.
Mr Perdue nearly won first time out against Mr Ossoff in November, falling just short of the needed majority with 49.7%. The other seat had more candidates, with 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 will be 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.
Both attended rallies on Monday evening.
Mr Biden told voters in Atlanta: “Georgia, the whole nation is looking to you.”
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.
In Dalton, President Trump told voters that the Georgia runoffs were the “last line of defence” against the Democrats.
He told voters “the whole world is watching” and that this was “your last chance to save the America that we love”.
The president spent a lot of his speech repeating claims he was the winner of the presidential election – and unsubstantiated allegations of widespread voter fraud.
Republican officials are worried this could depress turnout in Tuesday’s vote, although Mr Trump played this down, telling voters to “swarm it tomorrow”.