It seems to me there are a lot of non-political forces which will influence people's vote. Here are what come to my mind, I'd be curious to see if people knew of others.

McCain - Loses votes to age discrimination, though that is also a political consideration.

Palin - Clinton's former supporters liking her because she's a woman.
Other voters liking her simply because she's a woman.

Obama - The "black" issue, racism which is barely under the surface, will cost a lot of votes, I believe. He'd be 10 or 15 points ahead in the polls if he were white. And I don't think that the prejudice will show up in the polls, folks won't admit it.
As a corollary, folks voting against him because of his middle name, suspecting he's a Muslim. (That should be an outrage, by the way. We have millions of Muslims in the US, good, decent citizens. The US was founded not upon the idea of "God" but for freedom of religion.)
Some people voting for him because he is black.

Biden - I don't see any non-political forces working either for or against him except that he's Roman Catholic. Don't know how that works with the voters any more, though the Roman Catholic Church has become politically active in condemning his stand on abortion. There is a way to do it non-politically, but when the Pope condemns the Democratic Party as "the Party of death" they've crossed the line.