AMES, Iowa—A slow-moving Republican presidential campaign came to life Thursday night in a candidates' debate that featured the fiercest face-to-face exchanges of the 2012 contest.

Former Minnesota Gov. Tim Pawlenty, who has lagged behind in national public-opinion polls, came out swinging: He jabbed front-runner Mitt Romney, mocking his wealth, and he criticized surging fellow Minnesotan Michele Bachmann for what he called her lack of experience and successes.

"She...