But here's perhaps the biggest issue complicating efforts to get a handle on where the race really stands: different assumptions that each party's pollsters are making about the demographic makeup of the electorate. Republicans are anticipating that the body of voters who end up casting ballots will be more like the 2004 electorate, heavily white and male. Democrats argue that 2012 voters as a whole will look more like the electorate of four years ago when record numbers of minorities and young people turned out.
The difference has meant wildly disparate polling coming from Republicans and Democrats, with each side claiming that it's measuring voter attitudes more precisely than the opposition.
Said Republican strategist Phil Musser: "The conviction with which both sides say they are on a trajectory to victory is unique."
Tuesday will determine which side is correct. For now, the gulf between the two sides' polling has made it difficult to judge which candidate is faring better in the six up-for-grabs states.
In the final hours of the campaign, national polls show a neck-and-neck race for the popular vote.
But it's the Electoral College vote that elects the president. In that state-by-state race, Obama long has had the advantage because he's started with more states — and votes — in his column, giving him more ways to cobble together the victories he needs to reach 270. Romney has had fewer states and votes, and, thus few paths — though victory remained within his reach.
Said Mo Elleithee, a Democratic strategist who specializes in Virginia: "A 1 percent shift in any demographic group in Virginia is the difference between Barack Obama and Mitt Romney being president. That's how close this election is."
Over the past month, Romney's standing in national polls improved following strong performances in the October debates, and he's strengthened his position in several states, including Colorado, Florida and Virginia.