The closest White House race for decades is reaching a thrilling climax as millions of voters put their trust in either President Barack Obama or Republican challenger Mitt Romney.
Early indicators on who will win may come once the results are known in the key swing states of Ohio, Florida and Virginia. It's widely thought that Romney will need to win all three to stand a chance of an overall victory.
Perhaps crucially in Ohio exit polls gave him a slender lead of 51 to 48 percentages points.
In Florida the rivals were thought to be neck-and-neck with around 76% of the votes counted, with the prospect of an automatic recount rising by the second. If the margin of victory is less than or equal to 0.5% a recount will be called. At the moment it is being reported Obama is 0.1% in front.
Romney held an early lead in the third swing state of Virginia.
Obama's victories in the keenly fought battlegrounds of Wisconsin, Pennsylvania and New Hampshire made it even clearer that Romney would need to capture all three key swing states to get over the line.
And in a sure sign that the Democrats were becoming increasingly confident of victory the tense atmosphere at Obama's Chicago election night rally started to dissipate as crowds cheered each Obama win.
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Polls in the three crunch states closed at 1am for counting (GMT).
Down to the wire in FloridaFlorida voters wait in long lines to cast their ballots in the crucial battleground state as the 2012 presidential election goes down to the wire. Deborah Lutterbeck reports.
More than 30 million voters have already voted before polls opened, with over 30 states granting either absentee voting or early voting in person.
American TV networks projected Romney the winner in traditionally Republican states Indiana, Kentucky, West Virginia, Oklahoma, South Carolina.
Obama has already won the Democratic stronghold of Vermont and is the projected the winner of Maryland, Vermont, Connecticut, Delaware, Washington D.C., Massachusetts and his home state of Illinois.
But in a further blow to Romney CBS projected that Obama has bagged the swing state of New Hampshire.
Romney had high hopes he would carry the state's four electoral votes as he was formerly governor of the neighbouring state, Massachusetts.
[Live Blog: US Presidential Elections 2012]
Opinion polls before today's vote casting showed Obama and Romney neck-and-neck, although the Democratic incumbent held a narrow lead in several crucial swing states that could ensure the 270 electoral votes needed to emerge victorious in the state-by-state contest.
Around 120 million Americans were expected to register their votes today to determine whether Obama would earn a second term.
Obama and Republican Romney offer distinctly different policies to boost America's faltering economy, with Obama pledging to raise taxes on the rich and Romney promising wide-ranging tax cuts as a way to power the US forward.
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In the final 24 hours Romney made visits to Pennsylvania and Ohio to try and ramp up the turnout in those states, while Vice President Joe Biden was sent to Ohio. Obama stayed in his hometown of Chicago.
A bullish Romney told reporters on his aeroplane as he flew back to Boston he had written only a celebration speech.
He said: "I'm very proud of the campaign that I've run, to tell you the truth.
"I'm sure like any campaign, people can talk to mistakes, but that's going to be part of anything that'sproduced by human beings."
The former head of a private equity firm would be the first Mormon president and one of the richest ever to take on the presidential mantle.
Meanwhile, Obama told reporters he had speeches ready for either outcome.
Speaking to Denver television station FOX31, he said: "You always have two speeches prepared because you can't take anything for granted."
The nation's first black president is seeking to avoid serving just a single term - something that has happened to only one of the previous four occupants of the White House.