My guess is either Mark Warner or Hillary Clinton. The problem is that no democrat can raise as much money as Clinton during the primaries, where funding is crucial. Warner is the only democrat, due to his tech background, who has a donor base entirely separate from Clinton. The other candidates would be fighting for scraps. Moreover, Warner can present himself as the next bill clinton, a moderate southern governor who can win over red states. This argument would be powerful to those who don't think hillary has a chance in the general. If Clinton wins the nomination, there's a good chance she'll name wesley clark as her running mate. After all, she was the one who urged clark to run for president in 2004.