He was never really a Democrat anyway, he was an Independent. He just became a Democrat so he could run for POTUS. He's always been too Socialist for the Democrats but I think now the Democrats have caught up to him.
Bernie Sanders Leaves the Democratic Party
The nomination was barely sealed up at the Democratic National Convention before Bernie Sanders, who had campaigned against Hillary Clinton for the party’s nod, went back to being an Independent.
Sanders, who considers himself, officially, an Independent in Congress because his views lean further left than the Democratic party’s platform, caucuses with Democrats. But until declaring an intention to run for the presidency in 2015, he had rarely, if ever, identified as a member of the Democratic Party (he’s been in politics since 1979).
And now, despite pleading with his base to support Hillary, even though they’re concerned that she’s too moderate, Sanders will return to Vermont and to his seat in the Senate, and he’ll do it with no official party affiliation.
Debbie Wasserman Schultz, who was forced to resign as Chairwoman of the DNC after leaked emails revealed she’d tried to keep Sanders from challenging Clinton for the party’s nomination, might even be vindicated—sort of.
Sanders has struggled all along with whether to call himself a Democrat, even ducking the question of his party affiliation, raised by local Vermont media, just days after he declared. He later tried to reinforce that he was, indeed, a Democrat. But Sanders certainly wasn’t a party player—and that’s exactly the concern Wasserman Schultz voiced in the Wikileaks document dump.
In an April 24 email she received with an article describing the ways Sanders felt the DNC was undermining his campaign, she wrote back, “Spoken like someone who has never been a member of the Democratic Party and has no understanding of what we do.”
If Wasserman Schultz’s job was to ensure that a Democrat got the Democratic party nomination, then she might have been doing her job correctly (even if Bernie’s supporters would disagree).
There’s the additional complication, of course, that Wasserman Schultz was a vocal Clinton supporter, a Clinton surrogate and is now a senior adviser to the campaign, as she’s been officially booted from her DNC duties. But if anyone is vindicating her position, it’s Sanders, dumping the Democratic party as soon as it was no longer useful.