Former House speaker Newt Gingrich, facing increased scrutiny as he solidifies his standing atop the Republican presidential field, sought to convince conservatives in a forum Saturday night that he would not stray from a limited-government philosophy.

Asked why conservatives should trust that he won’t advance “big government approaches,” given his history of supporting an individual health-care mandate, climate change policies and a larger federal role in education policy, Gingrich said it was his years in Washington that made him best-suited to transform government.

“You cannot get the scale of change we want, and you can’t get the scale of change the tea parties want, by just appointing good people who have no understanding of the fight they’re about to be in,” Gingrich said.

Gingrich was one of six candidates who participated in former Arkansas governor Mike Huckabee’s 90-minute candidates’ forum on Fox News. At the insistence of the moderator, there were no clashes between the candidates. They appeared individually on Huckabee’s set in short segments and answered questions from three Republican state attorneys general.

Still, the hopefuls sought to draw distinctions between themselves as the Iowa caucuses are just weeks away and a new poll shows Gingrich pulling away from the field there.

A Des Moines Register poll released Saturday night showed that Gingrich had surged to 25 percent among likely Iowa caucus goers, while former Massachusetts governor Mitt Romney had slipped to third place, at 16 percent. The hotly anticipated survey had Rep. Ron Paul (Tex.) in second, with 18 percent.

The questioners — Virginia Attorney General Ken Cuccinelli II, Florida Attorney General Pam Bondi and Oklahoma Attorney General Scott Pruitt — are leading lawsuits challenging much of President Obama’s agenda, including the health-care overhaul and environmental regulations.

Romney avoided a direct contrast with Gingrich or any other candidate, instead highlighting his private-sector experience and differentiating his health-care law in Massachusetts with the federal overhaul. Romney said he would loosen environmental regulations that he said were stunting economic growth. The Environmental Protection Agency, Romney said, “is a tool in the hands of the president to crush the private enterprise system.”

The other candidates tried to find ways to appeal to the panel to establish ideological purity and a strict adherence to the Constitution with the party’s base.

Rep. Michele Bachmann (Minn.) said she was “very proud” of the three attorneys general for suing the EPA. And on the health-care law, she said: “It really is the new social engineering playground of the left, and it has to be stopped. . . . We have one chance to get rid of this bill and it’s in this election cycle.”

Paul cited the Constitution in advocating his libertarian position that violent acts are not the federal government’s prerogative. Asked whether he considered the Sept. 11 attacks to be terrorism, he said he did, but he argued that “more laws” and “federal policemen” are not the answer to fighting terrorism.

“We should be checking our borders and finding out who’s coming in, but we ought to understand that problem rather than saying what we need are more federal policemen,” Paul said.

Texas Gov. Rick Perry was more dramatic on stage. He pulled a copy of the Constitution out of his pocket, looked into a camera and said: “Read it. Exactly what it says. That’s what we’re talking about. Don’t read anything into it, don’t add to it, don’t use these different clauses . . . to try and change what our founding fathers were telling us.”

Seeking to rebound after his relatively moderate immigration record and poor debate performances set him back significantly, Perry cast himself as the best equipped to find a solution to illegal immigration. “I’ve been dealing with this for over 10 years now,” Perry said, pledging to secure the border in his first term as president.

Gingrich, too, has been facing scrutiny from conservatives over immigration. It was the subject of his first question, from Bondi, who asked about Gingrich’s proposal for neighborhood boards to weigh granting permanent residency to undocumented immigrants who have lived in the United States long enough to establish community roots.

“I’m suggesting that this only apply to people who’ve been here a very long time, who have a real tie to the local community, and we’re exploring the idea that they’d actually have a family sponsoring them,” Gingrich said.

He dismissed criticism that his proposal was a “magnet” drawing illegal immigrants to the country, saying that “a brand-new person looking to come to the U.S. for economic reasons” would be forced to enter legally.

Meanwhile, former Pennsylvania senator Rick Santorum offered himself as the strongest defender of traditional family values, saying that “the whole heart of American exceptionalism is about life.” As president, he said, he would start a national discussion about the divorce rate and out-of-wedlock birth rate.

“We’ve seen a federal government that’s undermined the family in a lot of ways,” Santorum said.

Businessman Herman Cain, who suspended his candidacy Saturday, and former Utah governor Jon Huntsman Jr., did not attend the forum.