The political pros say they'll be watching to see whether Pennsylvania goes red Tuesday - if so, Sen. John McCain might pull off one of the greatest comebacks in political history and be on his way to the White House.
But if Sen. Barack Obama holds Pennsylvania and collects Virginia or Indiana, two states that haven't gone Democratic since 1964, a landslide is in the offing for Democrats, and that could help them win down-ticket races that could give them a filibuster-proof majority in the Senate.
"If you think of the Electoral College, and Barack wins [Pennsylvania], it is difficult for McCain to overcome," said Rep. Gregory W. Meeks, New York Democrat. "If Pennsylvania goes to McCain, then it's a horserace. It indicates undecideds and independents are breaking his way."
In 2000, the bellwether was Florida, in 2004 Ohio and this time Pennsylvania.
"If it goes big for Obama, then you have the economy and the reverse Wilder effect," Mr. Meeks said, meaning that people who didn't want to admit to pollster - or to their friends - they were voting for a black Democrat actually did just that.
The final polls close in Indiana and Virginia at 7 p.m., and Pennsylvania closes its polls an hour later. In between is Ohio, which closes at 7:30 p.m. and which is critical to any Republican map.
"Pennsylvania is the bellwether state if I had to pick one, but I'd also say Ohio," said Tim Havrilek, a political consultant in Kentucky. "Democrats have an edge in Ohio, but Pennsylvania has a little of everything - mirrors the nation. It's the one to make a fair assessment for the rest of the nation."
Stephen Hess, an adviser to several presidents who is now at the Brookings Institution, said he'll be watching Virginia and North Carolina, but most of all he'll be looking at Indiana, which President Bush won by 20 percentage points in 2004.
In the Senate races, David E. Johnson, CEO of polling company Strategic Vision, said he will be looking at Georgia and Kentucky, where final polls close at 7 p.m. and where both seats are held by Republicans, Sen. Saxby Chambliss and Senate Minority Leader Mitch McConnell.
If both lose, Democrats will be in a good position to reach 60 Senate seats, which would give them a filibuster-proof chamber, denying Republicans their most powerful tool.
Of course, there are some critical unknowns that could ruin the predictions.
Elections officials are predicting historic turnout, with young and first-time voters making up a larger portion of the electorate. Pollsters have struggled to figure out how that will affect the actual vote totals, and Sen. Hillary Rodham Clinton's surprise primary win in New Hampshire this year, despite trailing in the final polls, was a cautionary tale to many.
Pollsters also wonder how much their surveys are skewed by the Wilder effect - named after the 1989 Virginia governor's race in which polls predicted an easy win for L. Douglas Wilder, a black Democrat who ended up winning by a slim margin.
On Monday evening, the McCain campaign fired off a memo from chief pollster Bill McInturff warning not to jump to conclusions from early exit polls, which he said have been skewed Democratic in the past and which have affected coverage even before the first polls close.
"The overstating of the Democratic vote did not only occur in the national exit polls, but also occurred in the state exit polls," Mr. McInturff said. "The 2004 exit poll report cited that the Kerry vote was overstated by more than one standard error in 26 states, while the Bush vote was overstated in just four states. So we should also expect the individual state exit polls on Tuesday to be more Democratic as well."
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