"The insurgent presidential candidate shifted further to the left last year in the run-up to the primaries, after ranking as the 16th- and 10th-most-liberal during his first two years in the Senate," National Journal notes.
Hillary Clinton was found to be tied as the 16th most liberal senator, after having placed 32nd the previous year. National Journal uses 99 "key Senate votes" to determine its ratings. One of the key "conservative" votes Clinton cast was to designate the Iranian revolutionary guard a terrorist organization.
The differences between Obama and Clinton might be smaller than the first appear, however: National Journal notes that "Of the 267 measures on which both senators cast votes in 2007, the two differed on only 10."
The ranking will likely be used against Obama if he becomes the Democratic nominee. On January 16th, Karl Rove had this to say: "Nonpartisan ratings say that he has a more liberal and a more straight-party voting record than Senator Clinton does. Pretty hard to do." Republicans are likely to question Obama's "unity" rhetoric by casting him as a far-left ideologue.
Interestingly, Sen. John McCain "did not vote frequently enough in 2007 to draw a composite score," National Journal writes. "He missed more than half of the votes in both the economic and foreign-policy categories."