Here is a key fact from our Massachusetts polling: even among Democratic voters in the state there are slightly more (25%) who think Congressional Democrats are too liberal than there are (24%) who think Congressional Democrats are too conservative.
Unhappiness with Democrats from the left seems to get more attention but the unhappiness from conservatives and conservative leaning moderates is a bigger threat to the party at the polls this year.
The Democrats who think Congressional leadership leans too far to the left voted 67-31 for Barack Obama in 2008. They express an intent to vote for Scott Brown by a 69-25 margin. I certainly agree that if health care and other major Democratic initiatives don't pass it could cause some of the party's base voters to sit on their hands this fall- but the bigger problem for the party is folks who think it's gone too far who aren't going to sit on their hands but will actually go out and vote Republican.