DES MOINES, Iowa » An intense debate over immigration flared among the Republican presidential candidates Wednesday as Mitt Romney declared that Newt Gingrich "offered a new doorway to amnesty" when he called for a "humane" immigration policy to avoid deportation for people who are deeply rooted in their churches and communities.

Romney, who is eager to stop the rise of Gingrich with the Iowa caucuses only six weeks away, signaled that he intended to go after his rival with the same vigor he used against Gov. Rick Perry of Texas two months ago when he said Republicans were "heartless" for standing in the way of offering education to children of illegal immigrants.

With the controversy likely to shape the next phase of the nominating fight, Romney repeatedly used the word "amnesty" during a campaign visit here to describe the position Gingrich outlined at a debate Tuesday night. While aides to Gingrich refuted the characterization of his plan as "amnesty," a backlash erupted among conservative activists that could present the biggest test of his resurgent candidacy.

Rep. Steve King of Iowa, a leading voice against illegal immigration, said he was puzzled that Gingrich had injected such a red-hot issue into his campaign. He said it was difficult to overstate how potent a problem it could be for Gingrich, saying it set off "a viral discussion among activists."

"When you have a campaign that’s ascending and you make a statement like that, it’s like you’re backing off on the throttle and diminishing yourself," King said in an interview. "It’s the same philosophy as the Dream Act. How many politicians have seen their campaigns end because of that?"

But Gingrich stood his ground and fired back at Romney in a message on Twitter, saying, "So what’s your position on citizenship for illegals again?"

His retort was a reference to a 2007 interview when Romney spoke favorably of creating a path toward citizenship for many of the 12 million people living here illegally. Asked about that Wednesday, Romney said that there was no discrepancy and added that he does not favor a "special deal" for anyone.

Gingrich’s advisers said that he did not misspeak at the debate and pointed out that his comments were in line with decades of positioning on the subject, including his support for the 1986 immigration overhaul signed by President Ronald Reagan that extended amnesty to about 3 million illegal immigrants. And he backed a less extensive overhaul in the 1990s as House speaker.

Gingrich, who takes Spanish classes and has started a bilingual website, The Americano, was not offering a new position. But his long-held view was suddenly receiving scrutiny because he has emerged as a leading candidate for the nomination.

But the strategy — sensible as it seems during a general election with independents to be courted — faces far different prospects in a Republican primary season, particularly in Iowa and other early-voting, conservative states.

"Iowa caucusgoers want a solution that does not include amnesty, and if they can paint Newt with an amnesty brush it will be toxic for his campaign," said Tim Albrecht, a top aide to Gov. Terry E. Branstad of Iowa.

While Gingrich said he supported securing the border and proposed creating an anti-fraud application system for immigrants, his plan to deal with those who entered the country illegally drew fresh notice. He suggested turning cases over to local citizens boards that could weigh whether residents could be allowed stay in the country.

"If you’ve been here 25 years, and you got three kids and two grandkids, you’ve been paying taxes and obeying the law, you belong to a local church," Gingrich said, "I don’t think we’re going to separate you from your family, uproot you forcefully and kick you out."

His comments touched off a torrent of criticism. Still, he took a reprieve from campaigning on the day before Thanksgiving, a sign that he was not worried.

But Romney, who is intensifying his efforts to win the Iowa caucuses, raised several questions about the plan put forward by Gingrich. He said Gingrich did not draw a distinction between someone who had been here for 25 years or had arrived illegally only recently.

"How about someone who has been here 20 years? How about 12 years? How about 10? Five? Three?" Romney said. "The real issue is, are we going to spend our time talking about how extensive we have amnesty?"

Gingrich’s spokesman, R.C. Hammond, pushed back against Romney, who also has vulnerabilities of his own on immigration.

"It isn’t hard to figure out what Mitt Romney is shoveling," Hammond said. "The facts show Newt’s plan is the opposite of amnesty."

Rep. Michele Bachmann of Minnesota pressed a similar line of attack in television interviews and in a statement from her campaign titled "Newt Gingrich’s Open Door to Illegal Immigrant Amnesty."

While she and other rivals have limited resources to spend on advertising campaigns, Romney has showed a willingness to play aggressively.

To respond to Perry’s support for a Texas law allowing in-state tuition for illegal immigrants, Romney sent sharply worded fliers across Iowa and organized teleconferences for voters with Sheriff Paul Babeu of Pinal County, Ariz., who has criticized Perry’s opposition to building a fence along the Mexican border.

While aides to Gingrich said that he would be able to explain his position, several Republican activists in Iowa said it could be an uphill battle.

"Some of the candidates will treat it like it’s an amnesty-type issue, which is a buzzword — a very negative word to a lot of conservatives," said Mark Lundberg, chairman of the Republican Party of Sioux County, one of the state’s most Republican regions.

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