A 2017 opinion-marked article in the Hill by Michael Starr Hopkins (himself apparently a Democrat) claims:

All the talk about building an economy that works for all Americans means nothing if “Bernie bros” attack every Democrat who isn’t Sanders. He isn’t even a registered Democrat.

It seems a little wired to claim that a Democratic Senator "isn’t even a registered Democrat". But is it technically possible, and was Sanders actually not registered as a Democrat when that piece was written?