During MSNBC's coverage of the February 9 Democratic primaries, MSNBC political analyst Pat Buchanan claimed that if Sen. Barack Obama (IL) wins the Democratic presidential nomination, Republicans will “tear him apart because ... he has the most liberal voting record in the United States Senate.” Buchanan did not cite a source for his claim -- presumably the National Journal's 2007 vote ratings; did not mention that the Journal changed its methodology after it rated Sen. John Kerry (D-MA) as the “most liberal senator” of 2003; did not touch upon the criteria used in the study; and did not note that a highly respected study by political science professors Keith Poole and Jeff Lewis ranked Obama as tied with Sen. Joe Biden (D-DE) as the 10th “most liberal” senator in 2007.

The Poole-Rosenthal system, developed by Poole and political science professor Howard Rosenthal and known as NOMINATE, has become widely used and cited among political scientists (see here for a list of academic studies that have utilized the Poole-Rosenthal system to evaluate legislative votes in both the U.S. and other countries). The Poole-Rosenthal ratings have a number of advantages over the National Journal ratings, most notably that they use every non-unanimous vote cast by every legislator to determine his or her relative ideology. By contrast, the National Journal's ratings were based on its own necessarily subjective selection of votes, what it describes as “a computer-assisted analysis that used 99 key Senate votes, selected by NJ reporters and editors, to place every senator on a liberal-to-conservative scale in each of three issue categories." Among the “liberal” positions Obama took to earn the distinction of “most liberal senator in 2007” were his votes to implement the bipartisan 9-11 Commission's homeland security recommendations, provide more children with health insurance, expand federal funding for embryonic stem-cell research, and maintain a federal minimum wage.

In a February 1 entry on The Monkey Cage, a political science blog, Brookings Institution senior fellow Sarah Binder wrote: “Because of the selective character of the NJ rating system and the unevenness of senators' participation, we should compare the NJ ranking to the 2007 ideological rankings produced by Keith Poole and Howard Rosenthal -- developers of the widely-used NOMINATE scores.”

Appearing with Buchanan on MSNBC, Air America Radio host Rachel Maddow noted that the "National Journal ranking ... conveniently in 2004 said that John Kerry was the most liberal senator in the United States Senate," adding: “You know I think that [Sen.] Bernie Sanders [I-VT], I think that Bernie Sanders, the independent socialist senator, should be suing the National Journal at this point, because they keep deciding that the Democratic presidential candidate is more liberal even than he. If that's the only thing they've got to throw at him [Obama], it's ridiculous.”

From the 8 p.m. ET hour of MSNBC's February 9 Democratic primary coverage:

BUCHANAN: I think Hillary clearly would run a stronger race. See, I generally believe that if Obama gets nominated, they're going to tear him apart because of his -- he's got to want -- you know, he has the most liberal voting record in the United States Senate. And Republicans know how to run against those candidates.

MADDOW: Oh, Pat --

BUCHANAN: Hold it, Rachel. Hold it, I can feel your hot breath coming through this. Now, on the -- there's no doubt on national security, national security and the war, Obama was against it from the beginning. No doubt about it; he's got a clear record. But McCain, that's all he's going to run on, see? McCain, that's what he really cares about. Immigration's almost off the table. And on that issue, national security, and “we're going to stay and fight and there's going to be no white flag of surrender,” that is perfectly tailored to go after Obama's case, who says “we're coming out, like it or not.” So you've got that issue right there. And frankly for Republicans, that's about the one issue they've got, since McCain has pretty much thrown immigration off the table.

EUGENE ROBINSON (Washington Post columnist): There are a couple of problems, in that McCain promises more wars. And I don't think that's what the American --

BUCHANAN: Yeah, he's got 100 years in Iraq. He'll have to back off from that a little bit, I think.

ROBINSON: I don't think that's what the American people want.

BUCHANAN: I agree.

NORAH O'DONNELL (co-anchor): What about the argument, Rachel? I mean, Barack Obama says that he presents the clearer contrast with John McCain on the war because, as Barack Obama has made his point, he opposed the war from the beginning. John McCain has supported this war. He has said that it was mismanaged, but he of course was one of the strong supporters of the surge and has said that we might be there for another 100 years.

MADDOW: Sure. I mean, Barack Obama would want to make the contest about Iraq to be about the decision to go in the first place. John McCain and, actually, Hillary Clinton both would like to make the discussion about Iraq be about what to do now and how to get out and whether or not to get out.

But, you know, when Pat mentioned -- and you heard me kind of scratching and clawing from over here about it -- when Pat mentioned this idea that Barack Obama is the most liberal senator, has the most liberal voting record in the Senate, that's from a National Journal ranking that conveniently in 2004 said that John Kerry was the most liberal senator in the United States Senate. You know, I think that Bernie Sanders, the independent socialist senator, should be suing the National Journal at this point, because they keep deciding that the Democratic presidential candidate is more liberal even than he. If that's the only thing they've got to throw at him, it's ridiculous.