Click here to watch "Folks."

The National Journal 2007 Senate vote ratings, released in January, attracted a lot of attention for ranking Barack Obama as the most liberal senator. Now they're front and center once again in John McCain's latest ad, "Folks" (subscription).

The ad wastes no time getting to the point. "Who is Barack Obama?" it begins. "The National Journal says he's the Senate's most liberal. How extreme." It then uses footage from two interviews Obama gave to show how he "defend[s] himself." In one clip, from an interview Obama did in September with MSNBC's Keith Olbermann, he says of the McCain campaign's advertising, "They're not telling the truth." In the other clip, from an interview conducted with the Christian Broadcasting Network in August, he says of those who attack his position on abortion, "I hate to say that people are lying, but here's a situation where folks are lying."

"Not presidential," says the ad's announcer. Repeating the clip of Obama saying "folks are lying," the ad tries to portray him as cagey and defensive.

It’s not the first time McCain has leveled the "most liberal" charge at Obama, but it is the first time an ad from the McCain camp has cited National Journal's vote ratings so explicitly as its central argument. In a column published during the Democratic convention, National Journal editor Charles Green explained the reasoning behind the ratings and responded to some of the criticisms that have been leveled against them.