While the most important trio of battlegrounds -- Ohio, Florida and Virginia -- remained too close to call, Obama grabbed Wisconsin, Michigan, Pennsylvania and New Hampshire, dramatically shrinking the Republican's chances.
At 10:45 pm (0330 GMT), US news network voting projections had Romney leading Obama by 184 to 173 electoral college votes, but the Democratic incumbent appeared to have a far clearer route to victory ahead of him.
As expected, Obama polled strongly in New England and the industrial northeast, grabbing densely populated states like New York, New Jersey and Wisconsin, according to news network projections.
His rival Romney did predictably well in the southern Bible Belt and western prairie states, winning the major prize of Texas and taking back Indiana, which Obama won in 2008, for the Republicans.
But Romney failed to win Michigan, his home state where his father served as governor, and the key battleground states of Florida, Virginia, North Carolina and Ohio remained too close to call.
Obama also won New Hampshire, which had been considered a swing state, and Wisconsin, the home state of Romney's running mate Paul Ryan.
Polling was due to have ended in the east of the country, but there were long lines in many areas and officials kept polling centers open late, as the campaigns pleaded with their supporters to stay the course.
The final opinion polls published before voting began showed the two candidates in a dead heat nationwide, but gave Obama a slight advantage in the handful of swing states like Virginia and Ohio that will decide the race.
Each state has a quota of electoral college votes based on its population, and the eventual victor will be the candidate who secures the most.
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