[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2008/images/09/18/art.bowave0918.ap.jpg caption="The McCain campaign claims Obama is the most liberal member of the U.S. Senate."]
The voiceover in a McCain campaign TV ad released in early September called Obama's "the Senate's most liberal." The ad included an image of the magazine National Journal, which labeled him the "most liberal senator in 2007." The "most liberal" claim has been repeated by McCain on the campaign trail.
Get the facts after the jump!
Facts: The National Journal magazine, in its annual vote rating, said Obama moved left last year to the "most liberal senator" rating "after ranking as the 16th and 10th most-liberal during his first two years in the Senate."
Americans for Democratic Action, the liberal activist group, and the American Conservative Union, the conservative activist group, also rate Congress members on their votes. Their findings describe Obama as one of, but not the most liberal U.S. senator.
The ADA gave Obama a 75 percent liberal score in 2007, 95 percent in 2006 and 100 percent in 2005. Other Democratic senators received 100 percent during those years. David Card, ADA communications director, said Obama's score was lower last year because he missed certain votes.
Obama has a lifetime ADA average of 90 percent. Other senators - such as Hillary Clinton, John Kerry, Ted Kennedy and others - have higher lifetime ADA ratings. Kerry, the Democratic presidential candidate in 2004, was ranked as the most liberal senator by the National Journal in 2003.
"He is one of the most liberal senators," Card said, referring to Obama.
The ACU, which customarily places conservative Republicans on the top of its list and liberal Democrats at the bottom, has given Obama a lifetime ranking of just 7.67, according to the figures on the group's Web site.
It says Obama scored 8 percent in 2005, 8 percent in 2006 and 7 percent in 2007. Other Democratic senators in the ACU rankings have had lower yearly and lifetime scores, the site shows.
"He's one of the most liberal," said Larry Hart, the ACU director of government relations.
Verdict: True, according to National Journal, but liberal and conservative rating groups reached a slightly different conclusion.