Oh, I have no doubts, looking at the list. But it's foolish to make a claim that X is a member of Y if both X and Y can shoot you down by simply denying it. I suspect these critters are way too careful to call themselves socialists (except for Bernie Sanders, and they'll elect anything in Vermont as long as it's queer, er, I mean odd. ) 'Progressive' doesn't have the same connotations, even though it means the same thing.
Sen Paul Wellstone is also a member of the Caucus or affiliated with them somehow or another.
Now living in VT and you've got that right. But I looked at the list from the link in post #15 and found my true home state of CO is just choked with these socialists/communists. Scarey, YES! And absolutely disgusting.