Barack Obama looks on course to secure a second term in the White House as Americans cast their votes following a \$2bn election campaign - the most expensive in US history.
The president appears to have the narrowest of leads over rival Mitt Romney in a number of critical swing states.
Voters have turned out in force, not least in Mr Obama's home town of Chicago where his supporters boldly predicted his re-election.
"Are you kidding? Obama," said Tim Glisson, 57, as he left a polling station in a school. "I voted for him because of his character, his fairness... just doing the right thing."
"First off, I'm never in my life going to be a Republican," said 64-year-old Sandra Rendrich. "Second, I don't think any president can get done what he need to get done in four years."
Republican Mr Romney and his wife Ann voted in Belmont, Massachusetts, before he headed to Cleveland in Ohio and Pittsburgh in Pennsylvania for a final round of campaigning.
Asked who he voted for, Mr Romney said: "I think you know," adding he felt "very, very good" about his election prospects.
In Cleveland he was joined by running mate Paul Ryan who said his boss was "running on fumes" after a hectic campaign schedule in recent days.
The pair stopped for lunch at Wendy's, where Mr Romney apparently ordered a quarter-pounder, chili and a Frosty.
Mr Obama, who voted earlier in the campaign, has spent the day in his home town of Chicago meeting volunteers and phoning to thank them personally for their help. He will later host a results party.
"I'm looking forward to the results, and I expect that we'll have a good night," Mr Obama told reporters.
"But no matter what happens, I just want to say how much I appreciate everybody who has supported me, everybody who has worked so hard on my behalf.
"And... I want to congratulate governor Romney and his team for a hard-fought race as well."
Vice President Joe Biden cast his ballot in Greenville, Delaware, saying: "It's always a kick."
When asked if he thought it was the last time he would vote for himself, he told reporters: "No, I don't think so."
Mr Biden shook hands with and hugged other voters as he waited. He urged Americans to vote "even if you have to stand in line". He later made a surprise visit to Ohio.
In a possible sign of how tight the race is, the first ballots on election day saw both candidates receive five votes each in the tiny town of Dixville Notch, New Hampshire.
The president wrapped up his electioneering with a rally in Iowa, the state where his 2008 campaign sparked into life, urging calling on Americans to give him a second chance.
"I came back to ask you to help us finish what we started because this is where our movement for change began, right here," he said.
"After all we've fought through together, we cannot give up on change now. We know what real change looks like."
:: Watch full coverage on Sky News as the results come in
The two candidates have criss-crossed a handful of swing states in recent days as they try to energise supporters and secure every last vote. Both have sounded weary and hoarse at times.
The latest 'poll of polls' by RealClearPolitics puts Mr Obama on 48.8% and Mr Romney on 48.1%.
In swing states, Mr Obama had a 2.9% lead in Ohio and was ahead by slimmer margins in Virginia and Colorado. Mr Romney led in Florida.
Other key states include Iowa, Wisconsin, Nevada and Pennsylvania, where Mr Romney poured huge sums into a late run. Between them he and Mr Obama have spent more than \$2bn on their campaigns.
The electoral college system and the state polls suggest Mr Obama could be headed toward re-election partly due to his lead in Ohio, according to Ipsos pollster Julia Clark.
Winning the US presidency is not just about the popular vote count but obtaining 270 electoral college votes. They are allocated to each state based on the size of its population.
Ms Clark said: "Obama only needs a couple of these swing states and the data suggests that he'll win one or two of them."
But Mr Romney's advisers have dismissed those polls and believe they have the momentum.
At a rally in New Hampshire on Monday night, Mr Romney said: "Tomorrow is a moment to look into the future and imagine what we can do, to put that past four years behind us and build a new future.
"Walk with me. Tomorrow, we begin a new tomorrow."
The economy has been the key issue in the campaign and both candidates have been driving grassroots efforts to mobilise support.
Mr Romney told them: "We have one job left, and that's to make sure that on election day, we make certain that everybody that's qualified to vote gets out to vote."
If elected, he would be the first Mormon to become US president.
In the last few days of campaigning, Mr Obama was accompanied by stars including Bruce Springsteen and Jay-Z, with former president Bill Clinton putting in appearances.
Both campaigns have victory rallies lined up, Mr Obama in Chicago and Mr Romney in Boston, where he has his HQ. The candidates will address the nation once the results are known.