It's not my idea, but from a comment (here, I think) I read in the last day or two that evidently stuck in the back of the mind for a bit, fermenting.
But it's a great idea, which taps a political reality that needs exploring.
The proposition, then, is that the Dem Congressional leaderships - in a thoroughly counterintuitive way - do not want to find themselves at the start of the 111th (which we assume will give the Dems the trifecta) with the kind of majorities that some have been suggesting are possible, what with the general disenchantment with the GOP, an above-average tally of open GOP House seats and a sophomore surge helping the 06 freshmen.
Let's run through the arguments supporting the proposition:
The vote today condemning the Moveon Petraeus ad is an excellent example of the Iron Law of Institutions-ie, that people within institutions act to increase their own power rather than the power of the institution itself.
A retarded baboon could understand it was in the long run best interests of the Democratic party as an institution to stand united against the bill. Yet 22 Democrats voted for it, thus passing it 70+ to 25.
Why? I guarantee you because in many cases the Democratic Senators don't like Moveon. It may be good for the party overall, but any new constituency usually creates enormous problems for those already in power. (At the least it requires you to spend time for their care and feeding.) Why would you want to change the status quo when you're the status quo? Far more appealing to take these interlopers down a peg.
We already know that Republicans are a gang of psychotic criminals. The lesson I'm drawing from this episode is any non-progressive Democrat may and often will betray us at any time.
My sense is that relations between Dems in Congress (particularly the Dem leaderships and other senior Dems) and the netroots (particularly the short head of the lefty sphere) are going to go through a bit of a bumpy patch over the fall.
I'm not sure there's anything much to be done about that; but at least I think we should try and understand what's going on.
The mutual incomprehension is partly ideological and partly generational (though we know that the netroots are hardly commie kids).