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Republican debate: Gingrich likens himself to Ronald Reagan. Republican front-runner Newt Gingrich likened himself to Ronald Reagan and insisted in a campaign debate Thursday night that he can defeat President Barack Obama in 2012, adding it was laughable for his rivals to challenge his conservative credentials.

Gingrich and former Massachusetts Gov. Mitt Romney — who runs second to Gingrich in the polls in Iowa — joined five other White House hopefuls on a debate stage for the last time before Iowa's Jan. 3 caucuses lead off the battle for the Republican presidential nomination.

The big question in the opening moments of a fast-paced two-hour debate went to the heart of a dilemma that could eventually settle the race — do conservative Republican voters pick a candidate who best represents their party's views, or do they simply want the candidate who is best able to defeat the president?

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Those voters begin making that choice on Jan. 3, and if experience is any guide, one or more of the presidential hopefuls on the debate stage will not make it out of the state to compete in the New Hampshire primary a week later.

Romney said his experience in private business made him the man to confront Obama in debates in 2012. "And I'll have credibility on the economy when he doesn't," he said.

The Republican primary winner faces Obama, whose favorable ratings have declined sharply, mostly because of his inability to unleash a strong economic recovery. Unemployment remains at a high 8.6 percent.

Gingrich, who seemed an also-ran in the earliest stages of the race, has emerged as a leader heading into the final stretch of the pre-primary campaign. He now leads the Republicans in the polls both in Iowa and nationally.

However, his conservative credentials were immediately challenged: Former Sen. Rick Santorum recalled that as House of Representatives speaker Gingrich had to contend with a "conservative revolution" from the ranks of Republican lawmakers.

Indeed, Gingrich is encumbered with a history of having left the House under a cloud of ethics complaints, and he has had three marriages and admitted marital infidelities.

Romney largely refrained from criticism of Gingrich, despite increasingly barbed attacks in day-to-day campaigning, including a characterization of the former speaker as "zany" for having endorsed mining the moon and lighting highways with mirrors in space.