AFTER a campaign that has cost an estimated $2 billion and deluged the nation with a million television ads, Americans are now facing the possibility of a result so close it will trigger lawsuits that could delay the result by days or even weeks.
At midnight on the US east coast, the first booth to be counted in the nation, the hamlet of Dixville Notch in the state of New Hampshire, which has 10 registered voters, recorded its first dead heat. Today, 700,000 votes will be counted in the wider state.
Voters get ready in Obama's hometown
RAW VIDEO: Mixed feelings for President Obama in Chicago. Some are predicting a victory for him, others just want the campaign and ads to end.
The Real Clear Politics poll average of national polls had US President Barack Obama up by 0.7 points on Monday night - a statistical tie - the CNN/Opinion Research poll had the two contenders literally tied, the ABC News/Washington Post poll had Mr Obama up 3 points, while Gallup found his Republican challenger Mitt Romney was ahead by 1 point.
Crucially, polls of the swing states also had Mr Obama ahead by a wafer in the dying hours of the campaign, and on Monday night some poll analysts were detecting a further late trend towards the President.
Because of the US electoral college system, a narrow victory - or even a marginal loss - in the popular vote could still lead to a comprehensive win for either candidate.
Either way both campaigns believe the result could, again, depend on the state of Ohio, with its 18 electoral college votes, where polls on Monday night found a range of results from a tie to an Obama advantage of 6 points, for an average lead of 2.9 points.
If the margin in Ohio is within 0.5 per cent, the state would automatically recount its ballots. Further complicating the Ohio ballot is that 1.3 million of its voters applied for absentee ballots.
Should those who applied for these ballots then turn up to vote on Tuesday, they will be allowed to cast a provisional ballot, and so far only 1.1 million have been returned.
Any provisional ballots cast may not be counted until November 17.
It is ''entirely possible'' the state could be forced into an automatic recount, says Robert Alexander, professor of politics at Ohio Northern University.
''Bush-Kerry came down to 119,000 votes in 2004 and Jimmy Carter beat Gerald Ford by just 11,000 votes here in 1976. The number of provisional ballots and absentee ballots may further cloud that picture,'' Professor Alexander said.
''The polls still show Obama ahead in the state, but the Romney camp is making a strong case that they think he can and will win the state.''
Romney in Ohio to squeeze out final votes
Republican Mitt Romney casts his ballot in Massachusetts, saying he feels confident about the battleground state of Ohio ahead of an unconventional election day trip to woo voters there.
David Beattie, a Democratic pollster in Florida, predicted litigation in his state and Ohio whatever the outcome, the Los Angeles Times reported. ''I would be shocked if there wasn't,'' he said. ''Democrats will see it as precedent for how future elections are held. And Republicans will do it if Obama's elected because they have nothing to lose.''
One suit has already been launched in Ohio over what a union calls the unlawful restrictions placed on early voting by Ohio's Republican secretary of state, Jon Husted. An appeal over that matter will not be heard until Wednesday in Ohio.
In Florida, lawyers for both parties have been engaged for months, said Susan MacManus, a political science professor at the University of South Florida.
She said Democrats were claiming more minorities would have been able to vote had there been fewer restrictions on early voting, while Republicans were concerned about voter fraud.
Despite Mr Obama's narrow but consistent lead, there is no sign of defeat in the Romney campaign.
On Monday night in New Hampshire - the fourth state he had visited over the day - Mr Romney told an excited crowd: ''The only thing that stands between us and some of the best years we've ever imagined is a lack of leadership, and that's why we have elections.
''Tomorrow is a moment to look into the future.''
Mr Romney will continue campaigning on Tuesday, with stops planned in Ohio and Pennsylvania.
Mr Obama plans to play a game of basketball, an election-day tradition. He gave a final nostalgic campaign speech on Monday night in Des Moines, Iowa, where his presidential campaign began in 2008.
''I came back to ask you to help us finish what we started, because this is where our movement for change began,'' he said.
''You took this campaign and you made it your own and you organised yourselves block by block, neighbourhood by neighbourhood, county by county, starting a movement that spread across the country, a movement made up of young and old, and rich and poor, and black and white, Latino, Asian, Native American, gay, straight, Democrats and Republicans who believe we've all got something to contribute.
''It's out of my hands now. It's in yours. All of it depends on what you do.''