The Democrats' failure to run good campaigns, always keeping congress teetering between the two corporate parties, is part of their culpability. If they all ran on, for example, abolishing health insurance companies, and won (because the majority of the population actually wants universal health care), and got elected, suddenly they'd be in the awkward position of having to do something that their big donors don't want and not having Republican control of congress as an excuse. The Democratic party is controlled opposition. They allow a sprinkling of 'progressives' in the party because it makes for a better show.