John Sides reviews past elections:
Given the state of the national polls right now—a virtual tie—the historical record suggests that they will be very close to the eventual outcome, but could be off by 1 or even 2 points, depending on the vicissitudes of turnout. Given the historical lack of systematic bias toward either party or the incumbent party in close elections, it’s difficult to predict which “direction” the polls might miss this year, if they miss. The national polls tend to overestimate the frontrunner’s vote share, but with the national polls tied, there is no clear national frontrunner at the moment.
If this same pattern occurs in the state polls, then the battleground state outcomes should be narrower than the polls reflect—movement which might mitigate Obama’s apparent lead in Ohio [and] Romney’s apparent lead in Florida, for example.