It takes 270 votes in the 538-member electoral college to win the White House. The race is tight in Florida, New Hampshire, Ohio and Virginia, according to CNN.
Exit polls showed Obama has taken Vermont, Washington DC, Illinois, Maryland, Massachusetts and Rhode Island in the winner-take-all elections in the states. Romney won Indiana, Kentucky, Oklahoma, South Carolina and West Virginia.
Maine awards electoral votes by congressional district. Obama has won three out of four in the state, with the final not yet called.
Of all the states thus far called, only Indiana switched hands since 2008, when Obama narrowly won it.