WASHINGTON -- President Barack Obama won re-election Tuesday night despite a fierce challenge from Republican Mitt Romney, prevailing in the face of a weak economy and high unemployment that encumbered his first term and crimped the middle class dreams of millions.

The president sealed his victory in Ohio, Iowa, New Hampshire and Colorado, four of the nine battleground states where the two rivals and their allies spent nearly $1 billion on dueling television commercials.

The president is chosen in a state-by-state tally of electors, not according to the nationwide popular vote, making such "battleground" states -- which vote neither Republican nor Democrat on a consistent basis -- particularly important in such a tight race.

"This happened because of you. Thank you" Obama tweeted to supporters as he secured four more years in the White House.

Ultimately, the result of the brawl of an election campaign appeared likely to be the political status quo. Democrats won two more years of control of the Senate, and Republicans were on track to do likewise in the House.

Romney was in Massachusetts, his long and grueling bid for the presidency at an unsuccessful end.

The two rivals were close in the popular vote.

Romney had 45.2 million votes, or 49 per cent. Obama had 45 million, also 49 per cent, with 65 per cent of precincts tallied.

But Obama's laser-like focus on battleground states gave him the majority in the electoral vote, where it mattered most. He had 284, or 14 more than needed for victory. Romney had 200.

Yet to be settled were battlegrounds in Florida, Virginia and Nevada.

The election emerged as a choice between two very different visions of government -- whether it occupies a major, front-row place in American lives or is in the background as a less-obtrusive facilitator for private enterprise and entrepreneurship.

The economy was rated the top issue by about 60 per cent of voters surveyed as they left their polling places. But more said former President George W. Bush bore responsibility for current circumstances than Obama did after nearly four years in office.

About 4 in 10 said the economy is on the mend, but more than that said it was stagnant or getting worse more than four years after the near-collapse of 2008. The survey was conducted for The Associated Press and a group of television networks.

Democrats got off to a quick start in their bid to renew their Senate majority, capturing seats in Indiana and Massachusetts now in Republican hands.

In Maine, independent former Gov. Angus King was elected to succeed retiring Republican Sen. Olympia Snowe. He has not yet said which party he will side with, but Republicans attacked him in television advertising during the race, and Democrats rushed to his cause.

Polls were still open in much of the country as the two rivals began claiming the spoils of a brawl of an election in a year in which the struggling economy put a crimp in the middle class dreams of millions.

The president was in Chicago as he awaited the voters' verdict on his four years in office. He told reporters he had a concession speech as well as victory remarks prepared. He congratulated Romney on a spirited campaign. "I know his supporters are just as engaged, just as enthusiastic and working just as hard today" as Obama's own, he added.

Romney reciprocated, congratulating the man who he had campaigned against for more than a year.

Earlier, he raced to Ohio and Pennsylvania for Election Day campaigning and projected confidence as he flew home to Massachusetts. "We fought to the very end, and I think that's why we'll be successful," he said, adding that he had finished writing a speech anticipating victory but nothing if the election went to his rival.

But the mood soured among the Republican high command as the votes came in and Obama ground out a lead in critical states.

Like Obama, Vice-President Joe Biden was in Chicago as he waited to find out if he was in line for a second term. Republican running mate Paul Ryan was with Romney in Boston, although he kept one eye on his re-election campaign for a House seat in Wisconsin, just in case.

Voters also chose a new Congress to serve alongside the man who will be inaugurated president in January, Democrats defending their majority in the Senate, and Republicans in the House.

The long campaign's cost soared into the billions, much of it spent on negative ads, some harshly so.