To be entirely fair, the Republican party does a good job of criticizing themselves.

Consider that a lot of the discourse you're seeing is from people who already believe the democratic party is the lesser of the two evils they're probably going to vote for them regardless.

If you're engaging in a conversation with someone else, whose only tangible difference between the two of you is that one of you believes Dems are a-okay, and one believes that both parties are shit, the only real talking points they have are what the Dems do that aren't great.

Obviously I don't think this is every case, but I know that if I wasn't already primed to have THIS argument, that'd probably be the route it'd take.