McCain disses Bush
I think the idea of McCain as Kerry's running mate (AP) is a pipe dream at best (almost as crazy as Kos's idea of a Kerry/Brokaw ticket). But the fact that McCain would even publicly entertain the idea has got to be a big blow to Bush. McCain is probably one of the most respected politicians in America today (quite possibly the most respected after Giulianni). When the voters hear that one of the biggest Republicans around is publicly dissing his party leader (and it is most certainly a big diss) it will give a lot of the fence-sitters pause.
And its a big vote of confidence for Kerry.