Bernie Sanders is occasionally a Democrat; particularly, he was when running for President in 2016 and almost certainly will be if he runs for President in 2020; as a Senator he is “merely” an independent who caucuses with the Democrats, doesn't have the Democratic Party support candidates against him, and is the Democratic caucus’s ranking member of the Senate Banking Committee.
No, he didn't get to be the ranking member of the Banking Committee without personal connections with just about every Dem leadership figure that matters, especially in the Senate.
Now, he might have the moral fortitude to stand up for his public values despite such connections, but denying that they exist is ludicrous.