It's one thing to build a mental model of a quietly racist electorate. Under this hypothesis, McCain, a war hero and credible candidate, would see unnatural levels of support as voters who were turned off by Barack Obama's skin color pretended they were attracted to McCain's biography and political platform. But that's not evident in the polling -- or, if it is, it hasn't been enough to give McCain a lead. So instead, the model seems to be that a substantial slice of the electorate are prankster racists who have spent the campaign deceiving pollsters so that, come election day, they can trigger a McCain upset, destroying the credibility of random statistical sampling and really putting one over on liberals who thought this country would turn out for a black guy named Hussein. I guess that's possible, but it doesn't seem very likely.
Not to be a cynic, but that's not even close. What folks are worrying about is the voter who has some racist tendencies but, being a good American, finds them shameful and wrong, and suppresses them. One way by which to affirm one's not racist status is to express support for Barack Obama -- and through that affirmation you can persuade yourself, too, that you're not a racist. The question, then, is whether these voters will be able to pull the trigger in the ballot box. This is a battle of psychology -- the racist id going to war with the rationalizing ego and super-ego -- and it will rage mostly beneath the surface and in code. The voter whose id wins out isn't going to say to himself "I'm sorry, but I just can't vote for a Black guy", he'll say "I'm sorry, but I just don't trust the guy."
That's not to say that everyone who is voting against Obama because they don't trust them is engaging in justification for latent racism. The point is merely to show that we could see a noticeable effect from racism reflected in the final polls, without thinking that folks have just been playing tricks on the pollsters. The tricks are being played in their own minds.