Bowers argument in essence is that the majority of Ds at risk are non-progressives, that progressives have some shots at pick ups, and that a smaller D majority with a larger progressive component is better for progressives overall. I think this largely depends on how well progressives do in mobilizing progressive Ds to vote when they are discouraged by the outcome in 2008. And, as usual, I continue to think we are way too far out to do reliable polling. The current correlations and indicators on various questions and methodologies have proven less predictive, rather than more predictive, in recent years due to numerous factors -- shifting demographic of the electorate, impact of broadband, and changes in the media landscape all being contributing factors.
On the plus side, the Rs show a a continued ability to shoot themselves in the foot, as demonstrated in NY-23. Self-identified Republicans are a much smaller base of support, although a healthy number of independents are conservatives who can be counted on to support conservative ideological candidates. But if independents largely stay home, the advantage favors Ds at this point. And while the most recent poll had independents more motivated than Ds (and primarily motivated by anger, which is bad for incumbent party), that is against generic candidate rather than against specific candidates.
Guess we'll have to hold an election after all to find out.