President Obama began the general-election race with a base of 18 states plus the District of Columbia, totaling 237 electoral votes. Mitt Romney began with a base of 23 states, totaling 191 votes. North Carolina is tipping toward Romney and Nevada toward Obama, putting the president at 243 and Romney at 206. Assuming that the candidates win those states, that leaves the battlegrounds:

Florida (29 electoral votes): If Romney loses Florida, his chances of winning depend on sweeping nine other states, including Ohio and Nevada. However, the state has been considered challenging territory for Obama all year. It could come down to which campaign can get its base of voters to the polls.

New Hampshire (4): It is the smallest of the swing states, but it's almost evenly divided among the parties, so it could be a critical stepping stone. Obama won the state in 2008, but Romney -- who has a vacation home there -- is hoping for a home state advantage.

Virginia (13): A recent demographic transformation has shifted its electorate from reliably Republican to divided. Thousands of new residents, many of them diverse, younger and more educated, have crowded the sprawling suburbs, turning the state into a toss-up. Surveys give Virginia the distinction of being the closest battleground state in the nation, said

Ohio (18): Perhaps more than any other swing state, Ohio means everything. For Obama, capturing the state would radically trim his rival's options. For Romney, it represents the bulwark of Obama's Midwestern line of defense. If the president were to carry Ohio, he could win an electoral majority by adding Virginia or Wisconsin or Colorado, or by winning Iowa and New Hampshire. If Romney does not win Ohio, his path to victory would have to include Colorado, Florida, Virginia, Wisconsin and either Iowa or New Hampshire. But if he does capture Ohio, he could become president by taking Florida and Virginia and then just one other contested state.

Wisconsin (10): Romney's recent surge raises the chance he could be the first GOP presidential nominee to win here since 1984.

Iowa (6): Obama hasn't trailed in the poll average for more than a year, but here's a wild card: The Des Moines Register's surprise endorsement of Romney.

Colorado (9): Republican candidates carried the state in nine of 10 elections before Obama won in 2008. An expanding Latino population and an influx of young people have reshaped the landscape. The Colorado electorate is nearly evenly divided now.