For a simple reason: They oversample Democrats, and are even more likely to do so this year thanks to enthusiasm among The One’s cultists. Per Geraghty, 77 percent of Obama’s supporters say they’re willing to spend 10 minutes answering an exit poll questionnaire versus just 64 percent of McCain’s. No matter what’s happening on the ground, then, expect The One to have a large-ish lead when the data finally drops at 5 p.m. tomorrow. If he doesn’t, things could get interesting.

Even the networks don’t trust them in isolation anymore:

[T]he networks now follow strict rules that govern projections, examining not only exit poll data but actual vote tabulation and turnout information. NBC — which keeps its decision desk isolated from the calls made by competing networks — will only call a winner once its statisticians conclude that the chance of an error is less than 1 in 200. And no calls will be made until all the polls have closed in a state…

[E]xit polls are designed to provide a demographic portrait of voters, not to predict the winner of a close race. The early waves of data can be especially misleading because they do not necessarily reflect an accurate sample of the electorate.

This year, there’s another factor to take into account: the huge surge in early voting. To get a measurement of those voters, Edison/Mitofsky has been doing an extensive telephone survey in the last week, focusing on the 18 states with the highest concentration of absentee and early voting.

For all the precautions, the networks’ decision desks will still be analyzing the exit poll with some wariness. In both 2004 and 2006, the polls overstated the Democratic vote in many states. Because of that, Fox News stopped relying on the data halfway through election night in 2006, declaring it unreliable.

A similar problem cropped up during this year’s primaries, when exit polls often showed a greater lead for Sen. Barack Obama than the actual vote, decision desk directors said.

The exit polling firm’s being sequestered until the fateful hour — no cell phones or Blackberries allowed — so if you see a “leak” somewhere on the ‘Net in the early afternoon hours, “Don’t be hoodwinked. Don’t be bamboozled. Don’t fall for the okey-doke.” If you missed yesterday’s post about poll closings, read it now for scheduling purposes. First-round games in Indiana and Virginia begin at 7 p.m., followed by Ohio and North Carolina at 7:30.