This is unconventional wisdom. It is congressional races that tend to break for challengers when the undecided vote exceeds the gap between the two candidates. In presidential races undecideds tend to vote for the incumbent (I remember this from poly sci classes in college). I would venture to guess that the actual CW of undecideds breaking for the incumbent would hold even truer in a time of war.
If the undecideds haven't gone to kerry by now, with the months of 527's slamming bush, then they're never going to go to him.
He had his chance to make the sale. He had a slick approach and a chorus of people badmouthing the competition. But people just aren't buying.