Polls closed Tuesday in the U.S. election battleground states Ohio and Florida, with networks saying the race was too close to call.
Ohio, which has 18 votes in the U.S. electoral college, is virtually a must-win for challenger Mitt Romney, as no Republican has won the White House without carrying the industrial Midwestern battleground state.
Polls also closed in North Carolina, where Romney had been favored but the battle was said to be very tight, and West Virginia, which networks called for the Republican challenger.
In early results, TV networks projected Obama winning reliably Democratic Vermont while Romney captured Republican-leaning Kentucky and Indiana. Obama had won Indiana in 2008 but it never became a battleground this time.
Who Americans choose will determine the country's course for the next four years on spending, taxes, healthcare and foreign policy challenges like the rise of China and Iran's nuclear ambitions.
Each man offered different policies to cure what ails America's weak economy, with Obama pledging to raise taxes on the wealthy and Romney offering across-the-board tax cuts as a way to reignite strong economic growth.
National opinion polls show Obama and Romney in a virtual dead heat, although the Democratic incumbent has a slight advantage in several vital swing states - most notably Ohio - that could give him the 270 electoral votes needed to win the state-by-state contest.
According to Reuters-Ipsos Election Day polling, one in three Obama voters said the economy was the most important issue for them, while half of Romney voters agreed. Healthcare was the second most important issue for Obama voters and the budget deficit was second for Romney voters. Unemployment was third for both.
Three-quarters of both Romney and Obama supporters decided to vote for their preferred candidate before the October debates, according to the data.
The Romney side was encouraged by what was described as heavy turnout in Republican areas from Florida to Colorado.
Romney made last-minute visits to Ohio and Pennsylvania to try to drive up turnout in those states, while Vice President Joe Biden was dispatched to Ohio. Obama remained in his hometown of Chicago.
Romney told reporters on his plane as he flew back to Boston that he was optimistic.
"I'm very proud of the campaign that I've run, to tell you the truth," he said. "I'm sure like any campaign, people can talk to mistakes, but that's going to be part of anything that's produced by human beings," he said.