I think uninterested voters who say it are indeed thinking of the parties habits of acting for and on behalf of proprietary interests. Their complaints don't follow up with policy statements but with complaints about handing out favors to their mates.

I think that at a state level you get substantially different programs, but public and commercial journalists basically act like covering state politics is an unreasonable attack on their credibility, so it's quite true that state politicians act corruptly.