This would be a hard one to call. Bennet seemed a lot weaker a few months ago. He's still not terribly well known, but among Democrats who pay attention to state politics (i.e.: the ones who will be voting in the primary), he's managed to thread the needle, maintaining a relatively moderate voting record while signaling somewhat leftish tendencies (public option) to the activists. He's also managed to get a lot of the Democrats' major donors to back him.
That said, it's my impression that there are a lot of active Dems who feel that Bennet hasn't been there in the trenches with them over the years, while Romanoff has. And a lot of those folks still think Gov. Ritter screwed Romanoff when he didn't appoint him to the Senate. I'm guessing that a sizable chunk of the folks who have donated to Bennet would switch over to Romanoff if he signaled that he's serious about this.
It's an interesting choice by Romanoff. My guess is that Ritter is actually more vulnerable next year than Bennet is. But going after Ritter actually would split the party, while there just aren't that many loyal Bennet people out there, at least not yet.