[T]he initial results of the exit poll interviews have had frequent problems with non-response bias, a consistent discrepancy favoring the Democrats that has appeared to some degree in every presidential election since 1988. Usually the bias is small, but in 2004 it was just big enough to convince millions of Americans who saw the leaked results on the Internet that John Kerry would defeat George W. Bush. It didn't work out that way.
The resulting uproar led the networks, beginning in 2006, to hold back the data from their news media clients in a sealed quarantine room on Election Day until 5 p.m. Eastern time. The quarantine means that any numbers purporting to be "exit polls" before 5 p.m. are almost certainly bogus.
Sarah Kliff's advice on exit polls:
Keep an eye on the demographics. [Tom Jensen, director of Public Policy Polling] and [David Flaherty, CEO of the right-leaning Magellan Strategies] both say this is going to be one of the crucial things they’ll watch [tonight].
“Here in Colorado, for example, Latinos were 13 percent of the vote in 2008,” says Flaherty. “If it comes back at 5 pm, that they’re 18 or 20 percent of the vote, that would be a key observation. Then the folks in Chicago are going to be extremely pleased. If we see it at 9 percent, it might cause the Romney folks to start thinking that group isn’t as enthusiastic.”
Here’s Jensen’s take: “What I’m going to be looking out for are the demographics of who voted along racial and age lines. We Democratic pollsters think that the African American and younger voters will have as high turnout as they did in 2008. Republican pollsters are projecting enthusiasm won’t be there.”