By Steve Holland

DES MOINES (Reuters) - Republican front-runner Newt Gingrich, buffeted by attacks, gets his chance to fight back on Thursday at a debate that may help clarify who will win Iowa's looming contest to choose a presidential candidate.

Gingrich takes center stage in Sioux City at the last debate before Iowa's January 3 contest at an exhilarating time for him. He is leading national polls of Republicans pondering who they want to send to take on Democratic President Barack Obama in next year's election.

But with his success has come a barrage of attacks. Rivals Mitt Romney, Ron Paul and others have raised doubts about his conservative principles. Iowa airwaves are filled with ads trying to derail him.

Gingrich is showing some signs of fatigue among Republicans in this Midwestern state, an indication that they remain open to voting for someone else.

A Public Policy Polling survey this week said Gingrich's support had dropped several percentage points, leading Paul narrowly by 22 percent to 21 percent, with 16 percent for Romney and Michele Bachmann at 11 percent.

The 8 p.m. CST debate sponsored by Fox News gives Gingrich a chance to right the ship. He has excelled in debates thus far, which has allowed him to emerge from the pack as the key conservative alternative to the more moderate Romney.

"I think Iowa is going to be a challenge, because you have everybody firing off simultaneously in a relatively small (media) market," Gingrich told reporters on Thursday in Iowa City.

Republicans say the race in Iowa is volatile, with three weeks to go until the state holds the first U.S. nominating contest to choose a Republican presidential nominee.

"The Iowa caucuses are completely wide open, at an unprecedented level. The debates, combined with the lateness in visits and ad blitzes, have created a completely unpredictable scenario," said Tim Albrecht, spokesman for Republican Iowa Governor Terry Branstad.

Gingrich is stressing that he wants to run a positive campaign, and Republicans here like what they are hearing from him.

"Newt Gingrich is smart enough, he's tough enough, he's experienced enough to take on the establishment in the Republican Party and win," said Kevin McLaughlin, the Republican Party chairman in Iowa's Polk County.

Paul, a libertarian House member from Texas who would love to upset the conventional wisdom by winning Iowa, has been particularly critical of Gingrich. He has denounced him as a serial hypocrite, a Washington insider who made $1.6 million from troubled mortgage giant Freddie Mac, some of it right before the housing market collapsed.

Romney's campaign is assaulting Gingrich daily for past positions, including a Web ad on Wednesday that portrayed Gingrich as chummy with liberal Democratic Congresswoman Nancy Pelosi. Romney told the New York Times that "zany is not what we need in a president."

Romney has never competed fully in Iowa but would like to slow down Gingrich in the state and give himself a better chance of victory on January 10 in New Hampshire, where his big lead has been slowly eroding.

In many ways, a Paul victory in Iowa over Gingrich would be viewed as a win for Romney, who has sought to lower his own expectations in the state after losing it dramatically in his 2008 presidential campaign despite a spirited effort.

Still Gingrich, with his plethora of ideas and willingness to challenge Republican orthodoxy, is tapping into conservative hearts in Iowa in a way that may push him over the finish line in first.

A victory here would give him a boost in New Hampshire, and provide important momentum looking ahead to South Carolina on January 21 and Florida on January 31. Gingrich leads in both of those Southern states.

"From the voters I talk to, it's clear that he's captured their imagination and he's the candidate they believe can not just present the right message, which is critical, but also can win and govern effectively," said Will Rogers, who plans to volunteer for Gingrich after departing the campaign six months ago during a staff exodus.

(Editing by Todd Eastham)