Newt Gingrich fought back Tuesday against rival Mitt Romney and negative advertising as the Republican presidential candidates flooded the airwaves in Iowa, with much of their messaging aimed against the former House speaker.

The contenders and their supporters spent almost $1 million on ads in the state last week, more than triple the amount shelled out three weeks ago.

Gingrich acknowledged that the ads against him are working, with new polling data this week showing that his advantage over Romney and status as front-runner have largely disappeared.

With two weeks to go before the crucial Iowa caucuses, Gingrich tried to force a backlash against Romney and the negative ads sponsored by an independent political action committee supporting the former Massachusetts governor. Gingrich called on Romney to state publicly that the “super PAC,” known as Restore Our Future, should stop running the attack ads.

“I have a simple challenge for Governor Romney,” Gingrich said while visiting a plant that makes heavy equipment, on the outskirts of Ottumwa. “This PAC was created by his former staff and funded by his personal friends. If he wants to stop it, he can say it publicly.”

“I don’t object to being outspent,” Gingrich added. “I object to lies. I object to negative smear campaigns. And I object to things that the candidate himself refuses to support.”

The burst in ad spending in Iowa offers the best evidence that Romney is focusing on the caucuses after his earlier strategy of playing down the state.

Texas Gov. Rick Perry and his supporters also are spending heavily in an effort to regain momentum for his candidacy. And all the contenders are making Gingrich a target of their criticism, according to data on ad spending from Kantar Media.

“TV is reality,” said Ken Goldstein, the head of Kantar’s political division. “It’s the ultimate tell — if you’re spending money there, you’re taking it seriously.”

The advertising data show that Romney is spending twice as much in Iowa as he is in New Hampshire, which will hold its primary Jan. 10. When the spending by Restore Our Future is taken into account, the amount of money behind Romney is almost 10 times greater in Iowa than in New Hampshire.

Restore Our Future was the biggest spender in Iowa last week, running $385,000 worth of spots, enough to air ads 585 times, according to Kantar. That’s more than three times the amount run by Romney’s campaign, underscoring the central role super PACs are playing in this election season.

Romney’s team has criticized Gingrich through direct mail, but has not taken that message to the airwaves. Under new campaign finance regulations, Romney and his campaign officials are barred from coordinating with the super PAC, which does not have the strict limits on donations that the campaign must follow.

Perry and his PAC spent a combined $547,000 on ads last week, including $323,000 in Iowa. The Texas governor began spending heavily in the state before the other candidates, and that appears to have given him a boost out of the single digits in recent polls.

Steven Holt, 53, a retired Marine from Denison, Iowa, said Perry’s ads condemning government waste and President Obama’s “war on religion” prompted him to give the governor another look, after he had been debating whether to vote for Gingrich or Romney.

“It’s absolutely absurd that in certain places you can’t say ‘merry Christmas’ anymore and they’re calling it a ‘holiday tree,’ ” he said after hearing Perry speak at Cronk’s Cafe Restaurant and Lounge last week. “Christianity is under attack in the United States of America.”

The barrage of criticism against Gingrich included an ad sponsored by Rep. Ron Paul (Tex.) called “Serial hypocrisy,” which attacks the former House speaker as “the very essence of the Washington insider” for receiving money from mortgage company Freddie Mac and supporting a national mandate for health insurance.

The candidates have much on the line in the state.

“Newt has a make-or-break position in Iowa,” said Brad Todd, a Republican consultant who oversaw the GOP ad campaign on behalf of Sen. John McCain (Ariz.), the party’s presidential nominee in 2008. “He will have no momentum if he doesn’t win Iowa.”

Gingrich is embracing a strategy, he said, of fighting the criticism with a strictly positive message aimed at turning voters against those responsible for the critical ads.

“Unanswered negative ads work in the short run unless they bounce back and discredit the people who are running them,” he said. “And you now understand my entire strategy for the next two weeks. The state of Iowa must choose between the negative and somebody who wants to be positive.”

Gingrich was referring to a widely reported interview on MSNBC’s “Morning Joe” program Tuesday in which Romney said that super PACS are a “disaster” that have “made a mockery” of the campaign season.

But Romney declined to ask the super PAC financed by his supporters and run by his former political aides to stop airing television ads attacking Gingrich.

“It’s illegal, as you probably know,” he told host Joe Scarborough. “Super PACs have to be entirely separate from a campaign and a candidate. I’m not allowed to communicate with a super PAC in any way, shape or form. My goodness, if we coordinate in any way whatsoever, we go to the big house.”

“And you’re not talking about the White House,” Scarborough said. “No,” Romney replied.

Staff writers Philip Rucker and Sandhya Somashekhar in Iowa contributed to this report.