The most likely Democratic nominee is Bill Richardson. The Democrats know a few things.
First, nominating a Senator/VP is a bad idea. The last non-incumbent Democratic candidates to lose were: Kerry (Senator), Gore (VP), Dukakis (Far-Left), Mondale (Both Far-Left and a VP).
Second, they can't nominate someone far out of the mainstream like Hilary Clinton or Al Gore. They need to nominate someone who will put Republican states in play, not Democratic ones.
Third, the Democratic party will not nominate someone who doesn't appeal to a certain block of voters.
Using these three criteria the following candidates are eliminated:
Kerry (Senator, Liberal, white)
Clinton (Senator, Liberal) appeals to women
Gore (VP, Liberal, white) appeals to angry Moveon type wingnuts
Warner (Governor, Moderate, white and un-charismatic)
Edwards (unemployed ,Liberal, no base of support)
Bayh (Senator, Moderate, white and blase
)
Richardson (Governor, Moderate, appeals to Hispanics and SW voters)
Feingold (Senator, Liberal, appeals to mavericks)
So really the candidates with the best shots are Warner, Richardson, Clinton, and Bayh. Only Richardson meets the three criteria. A Richardson/Feingold ticket would be devestatingly effective in the Southwest and Midwest, both areas where the Democrats need help.