Barack Obama tweeted picture of himself with his wife Michelle shortly after being declared the winner

President Barack Obama won re-election to the White House tonight with a landslide victory over Mitt Romney, according to projections from most of the television networks.

Broadcast networks called the 2012 presidential election for Obama as he swept the map with wins in the swings states of Ohio, Iowa, Wisconsin, Pennsylvania, New Hampshire and Virginia while Florida still hung in the balance.

Obama seemed poised for a resounding electoral college win - despite predictions of one of the tightest finishes in history and the dogged insistence of Romney advisers that they were making gains all over the political battlefield.

Despite unemployment standing at 7.9 per cent and Obama performing very poorly during the first presidential debate, Romney was crushed nationally, though he might finish only just behind in the popular vote.

Minutes after his victory was announced, the President tweeted: 'We're all in this together. That's how we campaigned, and that's who we are. Thank you.' He also posted a picture of himself embracing his wife Michelle - and the post rapidly became the most popular tweet of all time.

Romney called Obama to concede defeat shortly before 1am, a few minutes before he was set to take the stage at his Boston headquarters to deliver his concession speech.