The triumph for Obama could help pave his way to re-election and spelled worrisome news for Romney's bid for the White House, as his campaign had pushed hard to prevail in the north-east state that carries four electoral votes.
Obama took two big strides towards re-election on Tuesday by blocking Romney's grab for Pennsylvania and Wisconsin, the first key states called in their bitter White House race.
Romney had made a late run at the solidly Democratic states, as his aides predicted a late wave would oust Obama (51) from the Oval Office after one term as he struggled to deal with a slow economic recovery and high unemployment.
A huge cheer rang out at Obama headquarters when television networks projected Obama would retain Pennsylvania and its 20 electoral votes.
Obama's Midwestern line of defence also appeared to be holding, as networks called Wisconsin, the first of a trio of Obama firewall states, also including Iowa and Ohio, in a blow to native son Paul Ryan, the Republican vice-presidential nominee.
In another blow to Romney, Obama also captured the north-east swing state of New Hampshire, which was de-facto home ground for the Republican as he has a holiday home there, and was governor of neighbouring Massachusetts.
Early in what was expected to be a long evening of vote-counting, CNN figures showed Obama had 147 and Romney had 158 of the 270 votes needed in the state-by-state electoral college needed to claim the White House.
The results so far declared left Obama with a much easier path to the White House than his rival and Romney appeared to need at least five of the remaining seven swing states, possibly including Florida, Ohio, and Virginia.
Obama appeared to be performing well in the key counties in Florida, in some cases matching his showing of his stirring 2008 White House race, likely powered by African American and Hispanic voters.
At Romney headquarters in Boston, a stony silence greeted news that television projections had handed Michigan, the state of his birth and where his father George was governor, to Obama.
The president, who made history by becoming America's first black president after a euphoric victory, was aiming to carve new precedent on Tuesday, 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 multi-millionaire former investment manager and Massachusetts governor was laying low in a hotel in Boston awaiting results.
Early results and evidence from key counties in swing states did not show any sign of the Republican wave that Romney had predicted, and rather appeared to confirm the predictions of the Obama campaign for high voter turnout.
House remains under Republican control
As expected, TV networks projected that Republicans would win the House of Representatives, and Democrats were favoured to cling onto the Senate.
In early calls in the presidential race, TV networks, handed the rivals the safe states they always knew would be in their column.
Both candidates had earlier marked time while voters dictated their fates.
Romney appeared caught up in the emotion of seeing his name on the ballot for president of the United States and also saw an omen in a huge crowd that showed up at a multi-story parking lot to see his plane land at Pittsburgh airport.
"Intellectually I felt that we're going to win this and I've felt that for some time," Romney told reporters on his plane.
"But emotionally, just getting off the plane and seeing those people standing there … I not only think we're going to win intellectually but I feel it as well."
While Romney penned his victory speech, Obama took part in his election day tradition of playing a game of pick-up basketball with friends, including Chicago Bulls legend Scottie Pippen, after visiting a campaign office near his Chicago home.
The president, who like a third of Americans voted before election day, congratulated Romney on "a spirited campaign" despite their frequently hot tempered exchanges.
"I know that his supporters are just as engaged and just as enthusiastic and working just as hard today. We feel confident we've got the votes to win, that it's going to depend ultimately on whether those votes turn out," he said.
"I think anybody who's running for office would be lying if they say that there's not some butterflies before the polls come in because anything can happen," the president added later in a radio interview.
CBS News, quoting early exit polls, said 39% of people approached after they had voted said the economy, the key issue, was improving, while 31% 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.
Where your vote matters
The election went ahead in New Jersey with thousands of people without power, and large areas devastated by superstorm Sandy which roared ashore last week killing more than 100 people.
Adora Agim, an immigrant from Nigeria, said the chaos shouldn't stop voting. "I have lived in a Third World country where your vote does not matter. It's nice to be somewhere where it matters," she said, in Hoboken, New Jersey.
The central message of Obama's campaign has been that he saved America from a second Great Depression after the economy was on the brink of collapse when he took over from Republican president George W Bush in 2009.
He claims credit for ending the war in Iraq, saving the US auto industry, killing Osama bin Laden, offering almost every American health insurance, and passing the most sweeping Wall Street reform in decades.
Romney sought to mine frustration with the slow pace of the economic recovery and argued that the president was out of ideas and has no clue how to create jobs, with unemployment at 7.9% and millions out of work. – AFP