CEDAR RAPIDS, Iowa - Republican presidential candidate Mitt Romney dived into his campaign's full-scale critique of rival Newt Gingrich on Friday, standing by top supporters who described the former House speaker as self-serving and mocking some of his ideas about science and technology.

In return, top Gingrich backers described Romney's criticism as a sign of panic less than four weeks until the Iowa caucuses begin the 2012 nominating contest. Gingrich has risen to the top of Iowa polls in the last two weeks and is leading in some other states, too.

While Gingrich kept to his pledge not to criticize his GOP rivals, he reignited criticism for being a loose cannon by referring to the Palestinians as being an "invented" people.

Romney said in Des Moines that he stood by former New Hampshire Gov. John Sununu, who described Gingrich on Thursday as "self-serving" and "anti-conservative" for criticizing a Medicare provision in House Budget Chairman Paul Ryan's budget.

"I fully support John Sununu," Romney said when asked about the comments during a meeting with the Des Moines Register's editorial board. The former Massachusetts governor poked at Gingrich's statements about mining on the moon and lighting streets with mirrors from space. He noted differences on immigration and environmental policy.

But Romney also hinted at their larger career differences, a theme that could come up during Saturday's nationally televised debate in Des Moines and in television ads before the Jan. 3 caucuses.

It was Romney's most thorough critique of Gingrich since the former Georgia congressman leaped past him to the top of national GOP preference polls and to the top of surveys in Iowa.

Gingrich's top backers in Iowa attributed Romney's comments to worry that Gingrich could beat him and weaken his chances of winning in the must-win New Hampshire primary, which comes a week later.

"What we're seeing from Mitt Romney in Boston is desperation and panic, and I think that's going to be very frustrating to people moving forward," said Iowa State Rep. Linda Upmeyer, Gingrich's Iowa campaign chairwoman. "That isn't what wins you caucuses or elections here in Iowa."

The back-and-forth was the latest in a quickly intensifying two-candidate race for the nomination.

On Friday, the Romney campaign released an Internet video criticizing Gingrich for his statements about the Ryan plan and quoting conservative pundits who questioned Gingrich's discipline. The campaign also released a list of statements from congressional and statehouse leaders from around the country saying Gingrich has been a poor leader.

As if on cue, comments Gingrich made about the Palestinian people that challenge U.S. policy emerged.

"We've had an invented Palestinian people, who are in fact Arabs and were historically part of the Arab community," Gingrich said in an interview recorded this week with the Jewish Channel. "And for a variety of political reasons we have sustained this war with Israel since the 1940s. And I think it's tragic."