Let me suggest a hypothetical to the Democrats (or anyone) here

[Adam Griffin]

It'd be much more interesting general election match-up with him than it would be in the primary. He would lose the primary, no doubt, but would enjoy sympathy within some corners of the party. In the end, he'd probably wind up with a two-way proportion comparable to Romney/Santorum (assuming no other relevant challengers, he'd get ~30% of the vote).

The general, however...I think you might have a substantial group of people who voted for Gore in 2000 and that haven't voted D since give him a second glance; pity voters for having the election stolen from him/realizing just how bad Bush made things. On the other hand, his persona and image has taken a far-left turn since 2000, so I'm not confident how many of this group he'd actually be able to reclaim.