More often than not, there's some sort of "draft candidate" at this stage of the race, someone who is hyped up around August and then gets in in September and all of that implodes. Wesley Clark, Fred Thompson, Rick Perry. (On the Republican side, the draft candidates were notorious for sucking the life out of any conservative candidates already in the race, never to recover, basically serving as spoiler to the eventual winner, who almost always turned out to be a disappointment.)
Any "big name" Democrat like Clinton or Obama would have to recognize that.