After Newt Gingrich renewed his call to run a "positive campaign," Mitt Romney is putting that pledge to the test -- unleashing a new ad that questions Gingrich's past cooperation with House Democratic Leader Nancy Pelosi on climate change and knocking his understanding of the economy. 

The new round of criticism from the former Massachusetts governor comes ahead of a Fox News debate Thursday where the Republican frontrunner is sure to face criticism from all sides, on a record and leadership style that spans decades and is increasingly coming under the microscope. 

Watch Fox News and FoxNews.com for the Republican presidential primary candidates debate on Thursday night at 9 p.m. ET.

The former House speaker, who has built a stronger lead in the GOP presidential race than any candidate to date, is trying to stay the course of running a positive operation. He instructed his staff to take the high road in a letter this week, urging them to "avoid initiating attacks on other Republican candidates." Moving to practice what he preached, the campaign then fired its Iowa political director after learning he had called Mormonism -- the religion to which two GOP candidates, including Mitt Romney, subscribe -- a "cult." 

But as Gingrich tries to maintain a pro-Republican, anti-President Obama message, the candidate he replaced at the front of the polls is turning more aggressive. He's knocking Gingrich as the kind of D.C. insider incapable of shaking up the economy, while digging up classic clips that Gingrich would rather see archived than aired. 

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"Unlike Speaker Gingrich who spent his life in Washington, I spent my life helping create jobs," Romney said on Fox News Wednesday. 

Romney has tried to cast himself as the more consistent, less mercurial alternative to Gingrich, a candidate known as much for his big ideas as his tendency to throw rhetorical firebombs even if they buck the party orthodoxy. 

Romney's camp on Wednesday went further, putting out a web video that described Gingrich as an unreliable conservative. The video used what is arguably the year's most eligible piece of footage for a GOP attack ad -- a clip of the climate change ad Gingrich once cut with House Democratic Leader Pelosi. 

After flashing the image of the two of them sitting on a couch, the ad said: "With friends like Newt, who needs the left?" 

Gingrich has said the Pelosi ad was a mistake. 

But Romney's freshly aggressive approach has made Gingrich's pledge of positivity tough to uphold. 

Gingrich fired back Tuesday after Romney called on the ex-speaker to return the $1.6 million he earned as consultant to Freddie Mac. "I would just say that if Governor Romney would like to give back all the money he's earned from bankrupting companies and laying off employees over his years at Bain, then I would be glad to then listen to him," Gingrich said. 

Mocking Romney's gaffe at the last debate -- where he bet Texas Gov. Rick Perry $10,000 he couldn't produce evidence to back up a claim about Romney's book -- Gingrich bet Romney $10 he wouldn't return the money he made at Bain Capital. 

But Romney, on Fox News, said Gingrich's challenge is not comparable and reveals his lack of understanding about the economy. 

"The speaker is way off on that," he said. "He doesn't understand the economy if he doesn't understand that sometimes businesses succeed and sometimes fail. ... To suggest that there is something un-American, that there is something wrong about investing in an enterprise that ultimately doesn't succeed bespeaks an extraordinary lack of understanding of how the economy works." 

And he renewed his call for Gingrich to return the money. 

In Iowa, the site of the Fox News debate, Romney has reason to step up his game. Recent polling has shown Gingrich building a double-digit lead in the state, with Romney and Texas Rep. Ron Paul jockeying for second position. 

Nationally, Gingrich continues to hold onto his lead but signs of weakness are starting to bare. 

A new Wall Street Journal/NBC News poll showed Gingrich pulling in 40 percent of likely GOP voters; Romney attracted just 23 percent. But when other voters were surveyed, the poll found more Americans would rule out voting for Gingrich in the general election than would say the same of Romney. 

A Reuters poll also showed Gingrich leading Romney by 28-18 percent, but holding up worse against Obama in a hypothetical general election match-up. 

Romney on Wednesday said voters have seen a lot of fluctuation in the polls and claimed the primary race "may go on for months." Referencing the general election polls, he said primary voters are "coming to the conclusion" that he's the candidate who can beat Obama in November. 

Romney has tried to suggest Gingrich's surge is similar to that of other candidates -- like Perry and Herman Cain -- whose leads have peaked and flagged in a matter of weeks. 

Pollster Scott Rasmussen told Fox News on Wednesday that "there appears to be some rising disenchantment with Newt Gingrich, because that's what's happened to all the frontrunners this year." 

But national polls show Gingrich has risen considerably higher than other "not-Romney" candidates this year. The fact that he kept and built his lead after Cain dropped out of the race suggests those supporters drifted in large part over to his camp rather than attaching to the campaigns of second-tier candidates. 

The debate on Thursday will still present a chance for those other candidates to build a second wind ahead of the Jan. 3 caucuses. Minnesota Rep. Michele Bachmann, at the last debate, went after Gingrich and Romney hard, while making an appeal to Cain supporters by repeatedly invoking his 9-9-9 tax reform plan. 

The Fox News debate in Sioux City, Iowa, will begin at 9 p.m. ET. Set to attend are Romney, Gingrich, Bachmann, Perry, Paul, former Pennsylvania Sen. Rick Santorum and former Utah Gov. Jon Huntsman.