Michigan

16 electoral votes

2008: Obama won by 16.5 percent

2004: Kerry won by 3.4 percent

Michigan was not even in the battleground state conversation for most of the campaign, and if Romney wins here, it means he's having a good night. Most recent polls have shown the president holding a lead in the mid-single digits here, but the outside groups backing Romney saw enough opportunity that they made major investments late last month. Michigan hasn't gone red since 1988, but it is Romney's birth state, and his father was the well-liked governor of the state and a top auto executive. Still, the fact that Romney chose not to visit the state in the final days of the campaign suggests his campaign is less than optimistic.

Wisconsin

10 electoral votes

2008: Obama won by 13.9 percent

2004: Kerry won by 0.4 percent

The Obama campaign has been unpleasantly surprised by the fact that it only leads by a small margin in Wisconsin, a state Mr. Obama won by 14 points four years ago. But while it hasn't gone red since 1984, Wisconsin has a history of close elections, and polls suggest Republicans have a clear edge in enthusiasm. It is also one of the few states where Romney's ground game is considered comparable to that of the president, thanks to the mobilization on the right around the failed effort to recall Gov. Scott Walker. If Wisconsin goes to Romney, so might the presidency.

6:00 p.m. AKST

Iowa

6 electoral votes

2008: Obama won by 9.4 percent

2004: Bush won by 0.7 percent

Polling here has suggested a close race, though a late Des Moines Register survey showed Mr. Obama building a small lead; Democrats have also led in early voting. The state launched Mr. Obama's then-improbable presidential bid four years ago despite an electorate made up of largely of white voters, a group that has broken against the president. Losing it would thus be a psychological blow, though the president has plenty of paths to 270 that don't go through the Hawkeye State.

Nevada

6 electoral votes

2008: Obama won by 12.5 percent

2004: Bush won by 2.6 percent

Polls suggest Mr. Obama has a bigger lead here than in any other battleground, despite the state's miserable economy; if he loses here, it's a very bad sign for the president. His lead is tied to the fact that he holds a big advantage among Latinos, who are 27 percent of the population; if he struggles here it means he is underperforming with a crucial voting bloc that is also a significant presence in Florida, Colorado and Virginia.