4:00 p.m. AKST

Polls close in 15 states and the District of Columbia - including in Florida, Pennsylvania and New Hampshire.

Florida

29 electoral votes

2008: Obama won by 2.8 percent

2004: Bush won 5.0 percent

This is the battleground behemoth, and if Romney loses here, he's in deep trouble. (Mr. Obama entered Election Day virtually guaranteed to win 237 electoral votes; winning Florida would put him at 266, just four short of victory.) The polls here have been tight, though Romney may have a small advantage; it's no surprise that the GOP nominee stumped in Orlando Monday. One wildcard: A nor'easter storm is expected to hit the state on Election Day, potentially depressing turnout.

Most locations actually close in the Sunshine State at 7 p.m. Eastern Time, but there are some locations in the Florida Panhandle that are in the Central Time zone, and thus close an hour later. Media outlets do not release exit poll information or early returns until all the polls close.

A big lead here for Romney after the close might not tell the story: Many Democratic strongholds here tend to report results later than the rest of the state.

New Hampshire

4 electoral votes

2008: Obama won by 9.6 percent

2004: Kerry won by 1.4 percent

Florida is the biggest battleground; New Hampshire is the smallest. But that does not mean it hasn't been strongly contested: Both candidates have campaigned here in the campaign's final days, in part because unlike in some early voting states, CBS News estimates that only about 10 percent of voters here will have cast ballots before Election Day. New Hampshire is also perhaps the quintessential swing state - the only place where President George W. Bush won in 2000 but lost in 2004. In a tight race it could all come down to the Granite State, which happens to be where Romney has a summer home.

Pennsylvania

20 electoral votes

2008: Obama won by 10.3 percent

2004: Kerry won by 2.2 percent

The state that could blow the electoral map wide open. For most of the campaign, Romney largely ignored Pennsylvania under the assumption it was out of his reach. But driven by recent polls that showed the state within striking distance, Romney and the outside groups supporting him invested in the state - and the candidate himself used his precious campaign time to campaign here on the final weekend. If Romney can steal a victory here, his odds of victory will improve dramatically; keep an eye on turnout in and around Philadelphia, which drove Mr. Obama's ten-point win here four years ago.

5:00 p.m. AKST

Polls close in 14 states - among them Colorado, Michigan and Wisconsin.

Colorado

9 electoral votes

2008: Obama won by 9 percent

2004: Bush won by 4.7 percent

Colorado is perhaps the closest swing state in the nation, and a place where both campaigns believe the election will be decided by which way suburban women break and whether Mr. Obama sees a repeat of the huge turnout among young people that drove him to victory four years ago. A loss here would be a big blow for Romney; it would not sting quite so badly for Mr. Obama, since he has many more paths to victory that don't go through the state.