Long story short … given optimistic assumptions (McCain wins 2/3 of white undecideds, 100 percent of third-party support collapses), the undecided vote is worth a net of about a point for McCain. Given what I’d consider to be more neutral assumptions, there’s no particular reason to think that the undecided vote favors him.
My guess is that the truth is somewhere in between and that this is worth, say, half a point for McCain. Even give him the full point if you like. This effect is probably smaller than that of the cellphone problem, from which there may be 1-2 points of cushion in Obama’s direction. If on top of that the polls are being overly conservative with their likely voter modeling, the numbers are more likely to be underestimating Obama’s standing than overestimating it.
I’m one of those people who just can’t wrap my mind around the idea that any voter who intends to vote but still has not made up their mind at this point — six days out — is truly “undecided.” I could have believed it in 2000, but today? I think that deep down inside, the people who describe themselves as undecided have actually made up their mind and for whatever reason they’re not saying it out loud. It could be something as simple as they’re a Republican or a conservative who is voting for Obama, or it’s that they’re still struggling with the race issue. Who knows. But, as Mr. Silver notes, they could cancel each other out, and this year I think the polling indicates enough of a percentage gap between Mr. Obama and Mr. McCain that the undecideds won’t matter.