President Barack Obama was re-elected for an additional four years in office on Tuesday night, defeating challenger Mitt Romney with most voters' minds focused on the nation's economy as the top issue.

About 60 percent of voters said the economy was the top issue, according to exit polls conducted by AP and a group of television networks on Tuesday.

"This happened because of you. Thank you," President Obama tweeted after television networks began projecting his win.

"We're all in this together. That's how we campaigned, and that's who we are," he wrote.

The Associated Press reported late Tuesday that President Obama had 284 electoral votes, more than the 270 needed for a win. Former Massachusetts Gov. Mitt Romney had 200.

At the time of the report several contested states had not yet been settled, including Florida, Virginia and Nevada.

The victory appeared in hand for Ohio, Iowa, New Hampshire and Colorado, the report noted.