(CNN) - Barack Obama has demonstrated his appeal to independent voters and even some Republicans as he campaigns for president, though a just-released study from the National Journal indicates the Illinois Democrat was the most liberal senator in 2007.
Chief rival Hillary Clinton held the 16th most liberal voting record last year, the non-partisan survey of 99 major Senate votes found.
The study also shows both senators have moved to the left compared to previous years. In 2005 - Obama's first year in the Senate - he was ranked the 16th most liberal, and he came in at number 10 in 2006. Hillary Clinton has long held a moderate voting record: she debuted on the list at number 25 in 2001, and has been as high as 34. In 2006, the New York senator was ranked 32.
Massachusetts Sen. John Kerry, now a supporter of Obama, held the same distinction in 2003, the year he spent campaigning for the Democratic nomination. His high ranking was later used by Bush's re-election campaign to paint Kerry as an out-of-touch liberal.
"Ted Kennedy is the more conservative of the two senators from Massachusetts," Vice President Dick Cheney often said on the campaign trail, citing the study. The Republican National Committee also ran an ad against Kerry called "risky" in which an announcer said, "John Kerry…The most liberal man in the Senate. The most liberal person to ever run for president."
Kerry called the rating a "laughable characterization," and disputed its accuracy, pointing out that he missed 37 of the 62 votes on which the survey was based because he was campaigning for president. The publication has since raised the number of votes a senator must take to be included in the study.
Obama may be able to make a similar argument. According to the study, he missed 33 of the 99 votes that constituted the analysis. Clinton missed 16 of the votes.
But the details of the study suggest the Clinton and Obama's voting records are not as far apart as they appear in the rankings. Of the 65 votes included in the study that both senators were present for, they only differed twice - on a measure that sought to establish an Office of Public Integrity to handle ethics complaints (Clinton voted no, Obama yes), and on a measure that sought to allow certain immigrants to stay in the united states while renewing their visas (again, Clinton voted no and Obama yes).
Responding to the study, Obama spokesman Bill Burton said, "Only in Washington can you get falsely attacked for being like Reagan one week and labeled the most liberal the next. The tendency of Washington to apply a misleading label to every person and idea is just one of the many things we need to change about how things operate inside the beltway."
The full study is set to be released in March. The National Journal also notes Sen. John McCain, who is criticized by conservatives for some of his positions, did not take enough votes last year to qualify for the survey.
- CNN Producer Alexander Mooney