I think there will be a number of Americans who will casting their vote "not for McCain" and just defaulting to Obama instead of voting for Obama.
What this number will be I don't know but I am pretty sure it is more than zero
This is what is disappointing about American politics. More often we are forced to vote for the person we hate the least as opposed to voting for the person we like.
I think South Park summed it up well: A decision between a turd and a douche.