I'm not sure I understand the question. I don't think the Democrats really have problems in the sense that leftist critique of the party imagines them, and are not a lesser evil but actually are very good. I think the Sanders/Moore/Nader critique is institution-blind. Because Joe Manchin in a very red state runs as a Democrat but votes in problematic ways at times, it is apparently a taint on the entire party. But because of the nature of the Senate, people like Manchin are the pivot points.
Ultimately, they are underpants gnomes:
Step A: Demand ideological purity on the left.
Step B:
Step C: Utopia.
The key to more liberal outcomes isn't Democrats who are more liberal, it is more Democrats who are liberal.