The US state of Georgia is going to the polls for a second-round vote that will decide whether President-elect Joe Biden’s Democrats control the Senate.
Mr Biden’s party needs to win both seats in the state’s runoffs to gain full control of Congress – and with it the power to push forward his agenda.
The Republican Party of outgoing President Donald Trump needs only to win one in order to retain the Senate.
Mr Biden said Georgians could shape the US for years to come.
Meanwhile, Mr Trump told voters it was their “last chance to save the America” they loved.
Republicans Kelly Loeffler and David Perdue currently hold Georgia’s two Senate seats. 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).
What’s at stake in Georgia?
The vote will decide the balance of power in the 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 on key issues such as healthcare and environmental regulations – policy areas 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 would 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.
How will the vote proceed?
Voting should last about 12 hours, ending at 19:00 local time (midnight GMT), although all those still in line to vote at that time will be allowed to do so.
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.
Generally, results come in quickly but if these races are close, it could take 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 Democrat 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.
How important is the black vote for the Democrats?
Georgia’s black community is more than double America’s national proportion, making up a third of the population.
Across America, nine in 10 black voters supported Mr Biden in the presidential election, according to a survey of more than 110,000 voters for the Associated Press.
In Georgia, voting rights activists like former gubernatorial candidate Stacey Abrams have played a major part in driving up black support for the Democrats and delivering the state for Mr Biden in November.
Candidate Raphael Warnock serves as the senior pastor of the Atlanta church where assassinated civil rights leader Martin Luther King Jr grew up and preached.
If elected, the Baptist preacher would be the first black person to represent the state in the US Senate, as well as just the 11th black senator in American history.
What have Biden and Trump said?
Both attended rallies in the state 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.”
In Dalton, President Trump told voters the Georgia runoffs were the “last line of defence” against the Democrats.
“The whole world is watching,” he said.
The president spent a lot of his speech repeating claims he had won 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”.
Joe Biden’s first big test
It’s just over two weeks until Joe Biden’s inauguration, but the first real test of his presidency is on Tuesday.
If Democrats pick up the two seats and forge a 50-50 tie in the upper chamber, it’s still far from certain that Biden will be able to enact the kind of sweeping legislation on the environment, healthcare and the economy that he proposed during his successful presidential campaign. The narrowness of the margin will ensure that any laws will have to be supported by centrists in his party, like Joe Manchin of West Virginia and Arizona’s two senators.
It will, however, give the new president a fighting chance at legislative accomplishments – and make it significantly easier for him to appoint the administration officials and federal judges of his choice.
If the Republicans hold on, then Democratic hopes will rest on the whims of Majority Leader Mitch McConnell and a handful of Republican moderates.
Is Trump still challenging the White House election?
Mr Trump – who is due to leave office on 20 January – said at his Georgia rally: “They’re not taking this White House. We’re going to fight like hell.”
He 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.
“If he doesn’t come through, I won’t like him quite as much,” he said.
Some Republicans including Kelly Loeffler have signalled they will raise objections to the presidential election result in the House and Senate, requiring a debate and vote. Senator Ted Cruz, once a staunch critic of the president, is now his major ally.
But with other Republicans saying they will not contest Mr Biden’s victory, the votes questioning it would not succeed.
In his speech, Mr Biden accused Mr Trump of “whining and complaining” about the election result rather than concentrating on the Covid-19 pandemic.
Over the weekend it was revealed Mr Trump had pressured Georgia’s top election official, secretary of state Brad 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 won at least seven million more votes than the president.