In the wake of Joe Biden's nomination, Fred Barnes drags out the National Journal 2007 Senate ratings to argue that Obama and Biden are the first and third most liberal sentors:
Once regarded as a centrist, Mr. Biden was rated by the National Journal in 2007 as the third most liberal member of the Senate. Mr. Obama was rated the most liberal. Neither has a record of bucking the wishes of liberal interest groups or promoting bipartisanship.
However, as I pointed out back in February, the National Journal ratings are seen as simplistic by political scientists who study voting in Congress. The far more respected ranking produced by UCSD's Keith Poole and UCLA's Jeff Lewis places Obama and Biden as the 11th and 10th most liberal senators (respectively) in the first half of the 110th Senate (2007) and as the 21st and 29th most liberal in the 109th Senate (2005-2006).
By contrast, Poole and Lewis rate the "maverick" McCain as the eighth most conservative senator in the first half of the 110th and the second most conservative in the 109th, so the comparison isn't actually as flattering as Barnes thinks (though see my previous post on the methodological problems posed by his inconsistent voting record).