SIOUX CITY, Iowa (Reuters) - Republican presidential candidates pounced on front-runner Newt Gingrich on Thursday to try to blunt his surge at the last debate before Iowa launches the 2012 U.S. election season.

Gingrich is in a tight race with rivals Ron Paul and Mitt Romney in Iowa less than three weeks before the state's Republicans decide on January 3 who they want as their presidential candidate. It is anybody's guess at this stage as to who will win.

Gingrich's main adversary was not former Massachusetts Governor Romney as anticipated, but instead it was Michele Bachmann, the Minnesota congresswoman who won Iowa's straw poll of Republicans in August and would like to score a surprise victory here.

Bachmann repeatedly tried to raise doubts about Gingrich's conservative principles and accused him of being a Washington lobbyist for accepting up to $1.6 million in payments from troubled mortgage giant Freddie Mac, which was at the heart of America's housing crisis.

"We can't have as our nominee for the Republican Party someone who continues to stand for Freddie Mac and Fannie Mae. They need to be shut down, not built up," Bachmann said.

In a standoff over whether Gingrich in his congressional past had supported late-term abortion, Gingrich said Bachmann had her facts wrong.

Already, Gingrich is showing signs of fatigue among Republicans in this Midwestern state, an indication that they remain open to voting for someone else as a barrage of negative ads and verbal punches takes a toll on him.

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

All told, Gingrich appeared to hold his own at the debate and Romney might have missed a chance to follow up on attacks he has been making against the former speaker in the media all week.

"I'm very concerned about not appearing to be zany," Gingrich said at one point, breezily making reference to a criticism of him this week by Romney.

A Reuters/Ipsos national poll this week shows Gingrich holds a 10-point lead, but that he would fare worse than Romney against President Barack Obama at next November's election.

ROMNEY EXPLAINS POSITION CHANGES

At this debate, Romney let his rivals lead the battle against Gingrich, and adopted an above-the-fray stance, trying to establish himself as a credible alternative to Democratic Obama.

Romney gave his best answer yet to questions about why he has changed positions on some key issues during his political career. He denied changing positions on gay and gun rights and said his stance now opposing abortion evolved over time.

"Experience taught me sometime I was wrong. Where I was wrong, I've tried to correct myself," he said.

Paul, who came into the debate with a head of steam and is challenging Gingrich for the lead in Iowa, stumbled in answering a question about Iran's nuclear ambitions.

He insisted there was no evidence to suggest Iran was attempting to develop a nuclear weapon and enrich uranium. A U.N. nuclear watchdog report last month said Tehran appeared to have worked on designing a nuclear weapon, and that secret research to that end may be continuing.

"There has been no enrichment in Iran," said Paul, an anti-war libertarian.

Bachmann called him on it.

"We know without a shadow of a doubt that Iran will take a nuclear weapon, they will use it to wipe our ally Israel off the face of the map," she said.

Texas Governor Rick Perry, who is hoping a 44-city bus tour of Iowa will allow him to rebound after a string of bad debates, compared himself to American football star quarterback Tim Tebow, who has managed to win a string of games for NFL's Denver Broncos despite some obvious deficiencies.

"I hope I am the Tim Tebow of the Iowa caucuses. There were a lot of folks who said Tim Tebow would not be a very good professional quarterback," he said.

Gingrich, who has emerged as the lead conservative alternative to the more moderate Romney, compared himself to the Republicans' iconic President Ronald Reagan. He scoffed at his rivals' attacks on him as "kind of laughable."

"I think people have to watch my career and decide," said Gingrich, ticking off a conservative record he said he built up as House speaker in the 1990s.

The fact that this is the last debate before the Iowa caucuses increased pressure on Gingrich's rivals to press the attack against him and try to raise doubts about him.

Former Pennsylvania Senator Rick Santorum noted that Gingrich was not well liked as House speaker.

"The speaker had a conservative revolution against him when he was speaker of the House," said Santorum, who also subtly raised character issues about the thrice-married Gingrich, saying, "We need someone who is strong in the political and personal side."