Clark could potentially won the military vote. That may not have been enough to flip some states like South Carolina, North Carolina, Alabama, Georgia, but may have allowed him to pick up Virginia and Florida.
The Democrats always do better in general elections when they have a southern blue-dog candidate for President. That's how the Democrats can swing voters and win elections. Indeed, the only Democrat presidents in the past 30 years have both been from the south. As stated by Sal Paradise:
I'm becoming more and more convinced that despite being nominated due to "electability" that Kerry was actually the least electable candidate.
I think that Kerry was one of the least electable potential candidates. I felt he lacked the common touch and had a feel of intellectual elitism - not in the sense that he necessarily *was* smarter, but that he thought he was smarter... He was sort of the stereotype of a New England liberal, and I think the Dems need someone more populist if they are to win an election - that's why Clark or Edwards would have been a better pick for them, in my opinion.