(Reuters) Front-runner Newt Gingrich came under sharp attack from rival Republican presidential candidates Thursday night at the last debate before Iowa launches the 2012 election season.

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

At a Sioux City, Iowa, debate, Gingrich's rivals quickly pounced on his receiving up to $1.6 million in payments from troubled mortgage giant Freddie Mac as evidence that he has profited as a Washington insider from an enterprise that was at the heart of America's housing crisis.

Michele Bachmann, hoping Iowa's evangelical conservatives will give her a surprise victory in Iowa, said she was shocked that Gingrich was being considered as a potential Republican presidential nominee after acting as a lobbyist for Freddie Mac.

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. Gingrich fired back that Bachmann's charge was simply not true, that he was not a lobbyist for Freddie Mae, and insisted, I did no lobbying of any kind for any organization.

Gingrich, who has emerged as the lead conservative alternative to the more moderate Romney, compared himself to President Ronald Reagan, revered by Republicans. 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.

Already, Gingrich is showing signs of fatigue among Republicans in the 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 was leading Paul narrowly by 22 percent to 21 percent, with 16 percent for Romney and Michele Bachmann at 11 percent.

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.

But Romney, who entered the debate having flung a series of charges at Gingrich, let his rivals do the work for him during the early portion of the debate and insisted he has the ability to create jobs.

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.

This came on a day when the state's Republican governor, Terry Branstad, voiced concerns about Gingrich's discipline.

(Additional reporting by Sam Youngman; Editing by Bill Trott)