The video: Within hours of Monday's debate in South Carolina, in which Rick Santorum boasted that he's run a relentlessly positive campaign, the GOP presidential hopeful unveiled a "scathing" new ad targeting frontrunner Mitt Romney. The ad, which will run in South Carolina, harshly contrasts Romney's record with President Obama's. (Watch the clip below.) "Obama supported the Wall Street bailouts. So did Romney," says the ad's forebodingly voiced narrator as dueling images of the politicians flash across the screen. "Obama gave us radical ObamaCare — that was based on RomneyCare. Obama's a liberal on social issues. Romney once bragged he's even more liberal than Ted Kennedy on social issues." The narrator concludes: "Why would we ever vote for someone who is just like Obama when we can unite around Rick Santorum?"

The reaction: This is an incredibly effective ad, says Mark Halperin at TIME: The "unflattering photos of Obama and Romney are classics" of the negative-ad genre. And there are "more hot-button issues packed in here than there are Romney sons." Plus, says Susan Duclos at Wake Up America, "the takeaway from this ad is powerful in its simplicity": "Voting for Romney is the same thing as voting for Obama." But wasn't it just yesterday that Santorum "whined about" Romney's negative ads? says Taylor Marsh at her blog. Nonetheless, this "terrific" ad shows how Republicans could have taken Mitt down. "Why these guys didn't go this route earlier just doesn't make any sense." Now, it's too late, and "Republicans are very likely stuck with the guy." Have a look: