Is Obama a Liberal?

"Barack Obama is the most liberal member of the Senate" and "Joe Biden is the third most liberal member of the Senate" are common talking points for McCain and associates nowadays. These statements (silly to anyone who follows the Senate) are based on a single flawed National Journal report about 2007 voting patterns (they said the same thing about John Kerry in 2004). The only reason these senators ranked so high was because they missed many votes while campaigning. In previous years, Barack Obama had ranked 10th and 16th, and Biden was even more moderate. The National Journal themselves has said that their vote rankings should not be used in such a way, and just stated:

Joe Biden's voting record has generally placed him among the center of Senate Democrats ideologically in National Journal's annual congressional vote ratings

A separate, more comprehensive study of the 109th Congress found Obama to be the 21st most liberal Senator. The study of the 110th Congress put Joe Biden and Barack Obama tied as the 10th and 11th most liberal senators, which was exaggerated due to missed votes. Obama has also coauthored several successful bills with Republicans Richard Lugar, Tom Coburn, Kit Bond, and Chuck Hagel, who are nowhere near the liberal end of the spectrum.
The McCain campaign has tried to push these false impressions about both Obama and Biden themselves, even as they claim that "Obama has voted in lockstep with President George W. Bush nearly half the time." That's pretty hard to do if he's the most liberal member of the Senate, isn't it? John McCain has been in the Senate with both men and should know better.