UPDATE: US President Barack Obama has swept to a second term by defying the undertow of slow economic recovery and high unemployment.Obama became only the second Democrat to win a second four-year White House term since World War II, when television networks projected he would win the bellwether state of Ohio where he had staged a pitched battle with Mitt Romney.
"This happened because of you. Thank you," Obama tweeted to his 22 million followers on Twitter as a flurry of states, including Iowa, which nurtured his unlikely White House dreams suddenly tipped into his column.
With a clutch of swing states, including Florida and Virginia still to be declared, Obama already had 275 electoral votes, more than the 270 needed for the White House and looked set for a comfortable victory.
There was a sudden explosion of jubilation at Obama's Chicago victory party as the first African American president, who was elected on a wave of hope and euphoria four years ago, booked another four years in the White House.
Romney's aides had predicted that a late Romney wave would sweep Obama from office after a single term haunted by a sluggish recovery from the worst economic crisis since the 1930s Great Depression and high unemployment.
But a huge cheer rang out at Obama headquarters when television networks projected Obama would retain Pennsylvania and its 20 electoral votes, and the party grew wilder as they called Wisconsin and Michigan.
The mood at Romney headquarters in Boston, however, had grown subdued throughout the evening as partisans stared at their smartphones.
Disappointed Republicans were seen leaving what had been billed as a celebration of Romney's expected triumph in central Washington.
Defeats in New Hampshire, where Romney has a summer home and Wisconsin, the home of Republican vice presidential nominee Paul Ryan, were especially sickening for Republicans.
Early signs were that the election, while a building triumph for Obama would do little to ease the deep polarisation afflicting US politics, as Republicans racked up huge margins in safe states, though struggled in battlegrounds.
Exit polls appeared to vindicate the vision of the race offered by Obama's campaign, when top aides predicted that Obama's armies of African American, Latinos and young voters would come out in droves.
Polls also showed that though only 39 per cent of people believed that the economy was improving, around half of Americans blamed President George W. Bush for the tenuous situation, and not Obama.
The president, who made history by becoming America's first black president after a euphoric victory, carved a new precedent today by defying the portents of a hurting economy to win a second term.
He awaited his fate in his hometown of Chicago, while Romney, a multimillionaire former investment manager and Massachusetts governor was laying low in a hotel in Boston awaiting results.
CBS News, quoting early exit polls, said 39 per cent of people approached after they had voted said the economy, the key issue, was improving, while 31 per cent said it was worse and 28 saw it as staying the same.
Voters were also choosing a third of the Democratic-led Senate and the entire Republican-run House of Representatives. But, with neither chamber expected to change hands, the current political gridlock will likely continue.
The US presidential election is not directly decided by the popular vote, but requires candidates to pile up a majority - 270 - of 538 electoral votes awarded state-by-state on the basis of population.
A candidate can therefore win the nationwide popular vote and still be deprived of the presidency by falling short in the Electoral College.