From the perspective of this Dem, there are two reasons for treating moderates different. One is simply composition of the party -- those polls suggesting that the share of Republicans that are really solid conservative is greater than the share of Dems that are really solid liberals. (Hence, less room for a Republican equivalent of the DLC to get traction.) Curious to know if you buy those numbers. So "the base" takes up more space and there's less counter to the "we're not pure enough" true believers.
Second, effects of being out of power. When you were down as far as we were, people became a lot more willing to accept a Ben Nelson. Lieberman got different treatment, true, but part of that was the perception that he didn't need to be like that and that CT should be represented by a real Dem (whereas a "real" Dem wouldn't have much chance in Nebraska). Key grassroots leaders like Kos advocated for accepting less reliable Dems in some places. Why? 'Cause they were fed up with losing. Give the Republicans another couple of elections and they'll happily start accepting some moderates again. I'd say it'll be the '14 cycle (and it'll probably work for them then).
A long-time Democratic Hill staffer joins the debate: