He is the most liberal Senator, but that’s like saying the “most chaste prostitute.” Though the Democrats have the majority, there aren’t very many liberal Democrats in the Senate.
I found this - it’s pretty interesting to read.
http://www.nationaljournal.com/voteratings/sen/lib.htm?o1=lib_composite&o2=asc#vr
There certainly aren’t any conservative dems in there, either.