During a political campaign against any member of Congress, candidates and their supporters are fond of describing the opponent as either "The Most Liberal Member of Congress" or "The Most Conservative." Neither is usually true but, hey, when you run for office, you're allowed to make up just about any crap to describe the person you're running against. If you're interested in a non-partisan ranking (non-partisan in that the folks doing the scoring have no particular ax in need of grinding), here you go. Check out the voting records of the previous Congress and the previous make-up of the Senate. Needless to say, despite claims to the contrary, Barack Obama did not have the most Liberal voting record. He was thirteenth.