#2 most liberal Senator is apparently someone from Rhode Island named Sheldon Whitehouse.
http://www.nationaljournal.com/voteratings/sen/lib_cons.htm?o1=lib_composite&o2=desc
Never heard of the dude, myself.
The only thing I know about Sheldon is that he beat Lincoln Chafee in 2006. Lincoln must not have been Liberal enough for the constituency. It is also funny that the one self-proclaimed Socialist in politics, Bernie S., is not even in the top three.