When it comes to whipping votes (getting the party members to vote along party lines) only he and Angus King have the ability to claim no affiliation to the Democrat ticket, but Maine is a much more centrist state than Vermont. The house does not currently have any third party members.

Since voting is what matters (not privately held views), i'd say that he is indeed the only socialist in washington. If you insist on nitpicking about personal views, i'd say that perhaps you could include some representatives from the bay area, portland oregon, vermont (particularly the college towns), but that's about it. Many people always assume that the politicians hold secret views (shockingly similar to the those of the people who tend to believe it), but i honestly see no reason why people assume this. If someone has the forum to spread views the truly believe in, i don't seem much reason why they wouldn't.