The way I see it, the Kerry/McCain pairing would have two huge advantages that it could leverage over the American public.
1. They would represent a united front free of bi-partisan politics (whether it would work like that in reality, however, would be the question). McCain is not jumping ship; he would remain a Republican. As far as I know, this hasn't happened since Lincoln/Jackson. Either way, this would be a huge draw for a voting population that remains fairly moderate.
2. Both Kerry and McCain are 'nam vets. They can use that status as a giant hegemonic club to pretty much bludgeon anyone who gets in their way into silence.
This potential pairing spells a lot of trouble for Bush/Cheney. Currently, with the economy on a slide with no real viable economic plan, and the public becoming more and more critical of his "war on terrorism," the only card that Bush holds is his conduct after the 11/9 events. If it was just Kerry and some shmuck, I believe that he could have easily used his everyman charm, his father's influence, and the 11/9 attacks to pretty much muscle through the next election. But with McCain the scales tip in Kerry's favor drastically.
But I am no political forecaster. That being the case, I am interested in hearing the opinions of others in this matter, especially those that would be more educated in this subject than I am. Anybody here that's studied political theory or some such?