Republican rivals zeroed in again on Mitt Romney in the South Carolina debate tonight as they made a last-ditch, all-out effort to make certain he doesn't run away with the GOP nomination. They challenged his conservative credentials, his ability to create jobs, his flip-flops—and called on him to release his tax returns, notes the New York Times. A plain-spoken Newt Gingrich scored points with the audience, but Romney remained largely unflustered, and kept his eyes on the Big Race. "I'm the guy who can best post up against Barack Obama," he boasted. In other parrying: