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All year, debates have been the fulcrum of the GOP presidential primary race. But the pair of Iowa debates over the coming week, beginning with Saturday night's in Des Moines, will be the most pivotal yet, as the candidates gear up to make their closing arguments to caucus voters.

The spotlight — and the bullseye — will be on Newt Gingrich, the fragile but surging Republican front-runner. The former House Speaker is likely to face a fusillade of attacks from opponents desperate to knock him off his perch atop the field. This week's ad wars in Iowa presage a pile-on: Gingrich was the target of a blistering Ron Paul ad, and over the past two days Mitt Romney's team has emerged from its defensive crouch to attack Gingrich on a variety of fronts in an effort to sow doubts about his conservative credentials, character and long career in Washington. So far Gingrich has lived up to his promise to stay positive. Whether he sticks with that strategy in the face of an onslaught is an open question.

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For Mitt Romney, the challenge is to undercut Gingrich without driving up his own negatives by appearing too nasty. Romney doesn't have to win Iowa, but he does need to slow the former House Speaker's momentum; a double-digit caucus loss would be an ugly result for Romney, who's facing daunting Gingrich poll numbers in South Carolina and Florida. Fortunately for Romney, he'll have help in his effort to stop Gingrich: Ron Paul is certain to keep going negative, and each of the second-tier candidates — just six will be on stage with Herman Cain out of the race and Jon Huntsman's failure to qualify for Saturday's debate — may look to use their additional air time to confront him.

Of those candidates, the one to watch is Rick Perry. Debates have been a disastrous forum for the Texas Governor, but he is the only well-bankrolled candidate with a shot at getting the state's large social-conservative bloc to coalesce around his candidacy. Religious conservatives know that fracturing their vote would clear a path for Romney to eke out a win. And both Michele Bachmann and Rick Santorum are struggling to find a foothold in the state — though Santorum has lately picked up a couple of key endorsements — leaving Perry perhaps the best option.

"They truly want to stop Romney, and now that Gingrich is the frontrunner, that's the guy they want to endorse to do it," says an Iowa Republican insider, "but none of these guys can put their name behind a candidate who's been divorced three times." Perry's ad this week, which criticized Barack Obama's phantom "war on religion," pandered to the social-conservative cohort, which could still vault him back into contention as he gears up for a 44-stop bus tour of the state next week. But he'll need to avoid past mistakes on Saturday night; Republican voters will be watching.

READ: Campaign Countdown: 24 Days Till Iowa

• Alex Altman