Well, most Republicans will support Bush, just as most Democrats will support Kerry. That said, I think that Kerry will do well among the small percentage of "independent" Republicans, while Bush will not do well among the equivalent group on the Democratic side. Among self-identified Democrats and Republicans, each party has roughly 10% who are undecided. I think Kerry will get most of those votes.
Reality's mileage may vary. We may wake up in a week to learn that Bush will be prez for another four years. I'm cautiously optimistic though.