One of the most crucial battlegrounds, Virginia, is among the first states to end its voting, at 7 p.m. EST on Tuesday. The most vital prize of all, Ohio, closes its balloting shortly afterward at 7:30 p.m. EST.
Opinion polls show the race is tight in both states, and a winner might not be determined for many hours. But a stronger-than-expected showing by either candidate would be an early sign of which way the political winds are blowing.
"I wouldn't expect Ohio or Virginia to be called early, but if it starts to look clear one way or another that would be an early harbinger," said political scientist Steven Schier of Carleton College in Minnesota.
When Obama won Virginia in 2008, his victory margin there mirrored his 7-point win in the national race. Ohio is a traditional bellwether that is critical to Romney's hopes of reaching the 270 electoral votes he needs to capture the White House.
The Obama campaign will be evaluating turnout in selected precincts nationwide as the voting unfolds on Tuesday, but will look hard at the first actual vote totals from Ohio and Virginia, adviser David Axelrod said.
"We'll be focused on some of those Ohio precincts," Axelrod said on "Fox News Sunday," as well as on the early Virginia results. "If those start breaking our way it could be an earlier evening than people anticipated."
Romney's political director, Rich Beeson, said the campaign also will be looking for trends in the early numbers from the sprawling suburbs of northern Virginia and the heavily Democratic areas of northeastern Ohio.
They also will be watching the early results in New Hampshire and Pennsylvania, which are in the next wave of poll closings at 8 p.m. EST. New Hampshire is another swing state where polls show a tight race, and Romney has made a late bid to put Democratic-leaning Pennsylvania in play.
Beeson predicted a Romney win and said, "People won't have to look very long to know that."
The returns from New Hampshire, the smallest of the swing states, could be tabulated relatively quickly and provide the first swing state result. Florida, a must-win for Romney, also closes at 8 p.m. EST.
The closeness of the national polls has resurrected memories of the disputed 2000 election between Republican George W. Bush and Democrat Al Gore, which featured mistaken media predictions in Florida and a month of recounts and court battles.
Those memories could slow down the network projections of who will win individual swing states on Tuesday.
"There won't be a race to call states quickly. The networks are going to be very careful this time," Schier said.
Here is a rundown of the poll closing times for key swing states on Tuesday.
- 7 p.m. EST: Virginia.
- 7:30 p.m. EST: Ohio and North Carolina.
- 8 p.m. EST: Florida, New Hampshire, Pennsylvania.
- 9 p.m. EST: Colorado, Michigan, Minnesota, Wisconsin.
- 10 p.m. EST: Iowa, Nevada.