Assuming Ritter's decision was made on the basis of anything other than complete sincerity (a debatable point with him), he seems to have made the calculation that he's not going to get a serious primary challenger by next year (he's probably right about that) and that his path to reelection is moderation. Of course, at least with labor, he's getting a reputation as someone who doesn't dance with the gal what brung him. (Really, if your main issue is unions, how is Ritter an improvement over Owens?) Even if they don't run another Democrat against him, they could make his fundraising harder for next year, and a lot of them might just not turn out to vote for him.
Ritter and Sen. Bennet are in this curious little competition. Both are trying to portray a somewhat moderate image to improve their re-election prospects for next year, which may be a tough year for Democrats. But Bennet seems to be doing more to reach out to active Democrats (he kinda has to since they don't know him) and is at least vague on labor issues. I'm really curious which of them will have a higher vote margin next year.