I agree with holding withering fire right now, because the election is 6 months from now, and because Kerry is shooting himself in the foot pretty well. But, I don't think the end of the DEM convention (which I assume is the event you consider as marking "technically nominated") necessarily marks the winner as the person who GWB will run against.
The DEM party holds to no rules, and will switch Kerry out in a heartbeat once it is prevailing belief (stoked by media propaganda) that Kerry cannot win.
See Toricelli/Lautenberg. Expand it to the presidential level. Either they get away with it, or there will be, ummmm, well, errrr, lots of angry Democrats. Use your imagination.