I'd consider this debatable. He isn't even a member of the Democratic Party, he just caucuses with them.
1. Encourages people to vote for the democratic party.
2. Encourages people to run within the democratic party.
3. Was accepted and recognized as a candidate in the democratic primaries of 2016 and 2020 (even if the race was somewhat rigged against him).
4. Is endorsed by the democratic primary when he runs for Senatorship in Vermont.
5. Refrains from criticizing the leadership of the democratic party, even when their positions and policy are opposed to his stated positions.
6. Went on tour with DNC chairperson Tom Perez to convince people to support the party, after the 2016 elections.
7. If that's all not enough - he was a top appointed official of the party: Chair of senate outreach efforts as of 2016; see : https://www.usatoday.com/story/news/2016/11/16/senate-democr...
Long gone are the days when he was any sort of an outsider to the party.
I'd consider this debatable. He isn't even a member of the Democratic Party, he just caucuses with them.