I wish one of the Democratic candidates would just hit Sanders with his pro Soviet, pro Sandinista past now. The Trump ads for November write themselves and I'd rather see the public reaction now while there's time to replace him if it hurts too much.
There’s a Super PAC, The Big Tent Project, whose executive director is a top aide to Joe Manchin, that is spending millions airing aids against Sanders in Nevada and South Carolina calling him a socialist, etc. It’s not coming from an actual candidate, but the stuff will be out there. Their rationale was the same as yours: hit him with it now so that Trump can’t use it in November. That, and the donors behind the ads probably hate Sanders.