Special interests are corporations and superPACs. Establishment Democrats are slowly realizing the jig is up; they can't court both special interests and constituents anymore because constituents are realizing how politicians always represent big donors over little ones, case in point - universal healthcare, they can't choose only special interests because then they lose their voter base. The only option for success available is to abandon special interests and only accept direct contributions from individual donors by supporting a strong progressive message and actual progressive policy positions.
The only question in my mind is how many more elections will the Democratic leadership have to lose before they realize this?