Polls have begun to open in a tight US presidential race between Barack Obama, the incumbent, and Mitt Romney, his Republican challenger.
Voters in 25 US states, including battlegrounds New Hampshire, Virginia, Pennsylvania and North Carolina, were casting ballots by 7:30am eastern US time (12:30 GMT) on Tuesday.
The two candidates were on separate whirlwind tours of key swing states on Monday, with Romney's campaign events in New Hampshire lasting long into the night. Romney will continue to campaign on election day, making appearances in the key swing states of Ohio and Pennsylvania.
Barack Obama, the democrat incumbent, meanwhile, was in Iowa, where he began his first bid for the White House.
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The Iowa appearance was Obama's final campaign event, and he will be spending election day in his home city of Chicago.
Just after midnight on Tuesday, polls opened in the tiny northeastern villages of Dixville Notch and Hart's Location, in the swing-state of New Hampshire. The vote was tied 5-5 in Dixville Notch, while in Hart's Location Obama won with 23 votes to Romney's nine. Gary Johnson, the Libertarian candidate, also received a vote.
The two villages have enjoyed their first-vote status since 1948, but are not seen to be a national bellwether for the direction the election will go.
Both candidates have spent the past few weeks storming through a handful of key swing states .
"Tomorrow we begin a new tomorrow," Romney said at his Florida rally on Monday. "This nation is going to begin to change for the better tomorrow. Your work is making a difference."
Obama spent Monday emphasising the differences between his philosophy and Romney's.
"It's not just a choice between two candidates and two parties, it's a choice between two different visions for America," he said. He closed out his campaign with an appearance in Iowa, the state where he first launched his bid for the White House in the last election.
"I've come back to Iowa one more time to ask for your vote. I came back to ask you to help us finish what we've started, because this is where our movement for change began," he told a crowd of about 20,000 people.
Key poll closing times
With a system that is dependent on the number of electoral college votes a candidate can win, several battleground states have emerged as key players. These states could swing the election one way or another.
The unofficial result of the election is expected to become clear by 03:00-07:00 GMT on Wednesday.
- Virginia: 00:00 GMT
- North Carolina: 00:30 GMT
- Ohio: 00:30 GMT
- Florida: 01:00 GMT
- New Hampshire: 01:00 GMT
- Pennsylvania: 01:00 GMT
- Colorado: 02:00 GMT
- Wisconsin: 02:00 GMT
- Iowa: 03:00 GMT
- Nevada: 03:00 GMT
The two men both enlisted some celebrities for their final performances: Bruce Springsteen and Jay-Z headlined Obama's events, while Romney's featured Kid Rock and the Marshall Tucker Band, a country music group.
Stephanie Cutter, the Obama re-election campaign's deputy manager, told Al Jazeera "it's a close race". When asked whether Monday's closing arguments would sway voters, Cutter said: "We should know early by tomorrow where Ohio stands."
But even after months of campaigning, and billions of dollars spent to persuade voters one way or another, the race to the White House may yet drag on.
Patty Culhane, Al Jazeera's White House correspondent, has been travelling with the Obama campaign for the past few days.
"They're feeling pretty confident, they feel the electoral math is easier for them," she said. "But this is what all campaigns do. They try to build up a sense of momentum, of inevitability, but if you look at the polls, the president has a lead, but it's minor, and in most cases not beyond the margin of error.
"This race is simply too close to call. There's a lot of talk that this doesn't end tomorrow night, that this could continue. I can tell you that both of the campaigns have teams of lawyers in all of those important states because they're ready for legal challenges and recounts. In a race this close, it really could come down to that."
The last round of national polls were good news for the president. A Pew Research Center poll showed him leading Romney by three points, 48 per cent to 45 per cent. The same poll had them tied last week.
Two other polls showed a closer race: A Washington Post-ABC News poll had Obama leading by one point, 49 per cent to 48 per cent; and a CNN poll had the candidates tied with 49 per cent of the vote.
All three of those results were within the polls' margins of error.
But the popular vote will not decide the outcome. States are apportioned a number of electoral votes based on their population, and the candidate who wins a majority - 270 - becomes president. And the final state polls showed the president leading in most of the crucial swing states.
Surveys in Ohio have had Obama leading by anywhere from three to five points. A victory there would mean Romney would have to win at least six of the remaining eight battleground states, which seems unlikely: Obama led every poll conducted in Iowa, New Hampshire, Nevada, Virginia and Wisconsin; Romney's lone bright spot was North Carolina, where he looks poised to win by a narrow margin.
The other two battlegrounds, Colorado and Florida, seem too close to predict, with polls showing a range of possible outcomes.
Florida, arguably "the king of swing states" after the infamous election debacle of 2000 - which required the hand-counting of thousands of ballots - is facing new problems, with the state's Democratic Party filing a federal legal case over long delays encountered by some voters.
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The controversy erupted over "in person absentee ballots" that officials in Miami Dade County said voters would be allowed to cast on Sunday. But after being open for barely an hour, the Miami-Dade office closed its doors, saying they were swamped by the unexpected high turnout.
The decision to cut short the vote led to an uproar outside the government office, where the Miami Herald reported nearly 200 people began shouting: "We want to vote!" "Let us vote!"
"This is America, not a third-world country," Myrna Peralta told the daily.
In general, Democrats favour early voting, so limiting the number of people who vote ahead of the official November 6 election is seen as putting a thumb on the scale for Romney.
A judge in the central Florida city of Orlando on Sunday, however, ordered early voting to be extended in Orange County by several hours to accommodate the flood of voters.
Whoever wins, the next president is likely to have a difficult time pushing his agenda through a divided Congress.
Republicans are expected to keep control of the House of Representatives, the lower legislative body, though polls show them losing some of their 50-seat majority.
Democrats control the upper house, the Senate, by the narrowest of margins: They hold 51 seats in the 100-member body. Polls show them maintaining that majority, and perhaps picking up one or two extra seats.
Ayse Alibeyoglu contributed reporting from Washington.