The continental United States covers four times zones from east to west. Aside from Dixville Notch, the New Hampshire village whose 12 residents vote just after midnight Tuesday, the earliest polling stations open at 6:00am (1100 GMT) in Virginia and New Hampshire, among other eastern states. On the West Coast, California polls open at 1500 GMT.
Some polling sites in the east close at 6:00 pm (2300 GMT), but the first significant closures are an hour later in battleground Virginia. Everyone will be watching for early results in the state as a potential bellwether of the night ahead.
At 7:30pm (0030 GMT) polls close in North Carolina and all-important Ohio. A win for Romney in North Carolina, one of the more conservative swing states, would keep his hopes alive. But no Republican has won the White House without taking Ohio, and a loss there would put Romney in a massive hole.
Others will start to fall into place after 8:00 pm (0100 GMT), when the most populous swing state, Florida closes along with most eastern states. An Obama win in Florida would be monumental for his re-election hopes, as polls have shown the Sunshine State leaning to Romney in recent weeks.
West Coast states generally close three hours later.
This year, the nation's broadcast networks ABC, CBS and NBC, plus cable giants CNN and FOX, are conducting exit polls of some 25,000 voters, mainly in key states. Those figures, together with telephone polls and vote counts from precincts, will be used in formulating state predictions, which are made only after polls close.
Partial results will be posted by some states, and networks will show such results ahead of predicting the state's winner.
A candidate must win 270 of 538 electoral votes to clinch the White House. Eleven states, collectively representing a jackpot of 146 electoral votes, are up for grabs, according to RealClearPolitics. As a measure of how tight the race is this year, Obama won every one of these states in 2008. Of the 11, Obama's campaign says traditional Democrat states Michigan, Pennsylvania and Wisconsin are in his column. But Republicans have made late campaign moves there.
The Obama scenario
Based on recent poll averages, Obama is a lock in 18 states totaling 201 electoral votes. He has notable leads in Michigan (16 electoral votes) Pennsylvania (20) and Wisconsin (10). If Obama holds those states, he needs just Ohio (18) and Iowa (6) to win re-election. Or just Florida (29).
Look for Virginia (13) and North Carolina (15) as key early tests; if Obama wins one of them, it'll be a long night for Romney.
The Romney scenario
The challenger's path to victory is narrower. He is assured 24 states representing 191 electoral votes, leaving him 79 short. If Obama holds Michigan, Pennsylvania and Wisconsin, Romney must win two of the three biggest toss-ups -- Florida, Ohio and Virginia -- as well as most of the other battlegrounds.
Look for New Hampshire (4) as a key early test; it's small, but potentially indicative of how the night may turn for Romney.
All 435 seats in the House of Representatives are up for grabs, as are 33 seats in the Senate. Republicans are expected to hold the House. The Democrats' 53-47 majority in the Senate is more tenuous. A race to watch is the Massachusetts battle between Republican incumbent Scott Brown and Elizabeth Warren. Other key Senate contests are in Indiana, Missouri and Virginia.
More than 30% of Americans are expected to vote before Tuesday -- either absentee or in person.
Each state has its own recount rules. In the 2000 election between George W. Bush and Al Gore, some Florida counties launched recounts while others did not. With the prospect of very close results in some states, phalanxes of lawyers on each side are prepared to bring legal action, raising the potential for final result delays.
The Ohio question
A nightmare scenario may be brewing in crucial Ohio, where authorities sent absentee ballot applications to every voter. People who applied for such ballots but then decide to vote in person will be required to cast provisional ballots that are sealed until it can be proven that they haven't already voted.
Some 200,000 provisional ballots may be cast, and state law does not allow them to be opened until November 17.
Complicating the count are mail-in ballots, which can arrive as late as November 16 so long as they are post-marked by November 5.
And if the results are within 0.5 percentage points, an automatic recount of all ballots is triggered.