I still think the VP choices of the two Democrats who ran against W in 2000 and 2004 could have made all the difference. Not often is it the case that a VP pick is that decisive but those were the years where it certainly mattered most as they were both close elections. Gore/Graham & Kerry/Gephardt might have made all the difference in those years.
For 2000, I would certainly agree. Especially if Al Gore chose Bob Graham. I'm not so sure it would have made enough of a difference in 2004 given that election was nearly as close. Still close, but Dubya won by more than a hair this time around.
Both Lieberman and Edwards were weak choices though. No question about that.