I have stated several times on this blog that Bernie Sanders is not a Democrat. He may be running in the Democratic primary (and caucuses with Senate Democrats), but he shuns the party label -- and always has. He has yet to say in this campaign -- "I am a Democrat", and I doubt you'll ever hear him say that.
So, why is he running in the Democratic primary? He's just using the Democratic Party apparatus for his own political ends -- and he doesn't really try to hide that. Consider these posts from Politico.com:
March 1, 2016
This is a major reason why he will not get the Democratic nomination for president. Democrats know he is not one of them -- and they will nominate Hillary Clinton (who has a long and proud history of being a Democrat, and supporting Democratic issues and causes).