Problem is, the centerline for moderation has moved so far to the right in the last 20 years that we need responsible progressives to nudge it back where it belongs. Hilary Clinton was a habitual centrist and I think she actually suffered from it. The right likes to pillory Nancy Pelosi as a liberal, but she's takes money from the same lobbyists as the Republicans, and was willing to compromise along with the best of them back when this wasn't a binary system.
Bernie Sanders is a different animal. He's portrayed as an extremist for advancing ideas like single payer healthcare and free college tuition, but Sanders and others are able to back up genuine policy initiatives with surprisingly plausible cost/benefit numbers. Back in 2009, when Americans were polled on potential healthcare systems, 60+ percent thought Single Payer was the way to go --- because it was clearly explained to them. When opponents screamed Socialized Medicine!, a reasonable law that would benefit as many Americans as possible was suddenly marginalized as extremist.
If Democrats serve up a slate of moderates promoting their willingness to compromise, I think they'll lose the moment the midterms are providing. I think a big part of holding Democrats accountable is reminding them how uninspiring the party establishment has been.