DAVENPORT, Iowa – Facing slipping poll numbers, Newt Gingrich lashed out at his GOP presidential rivals Monday, calling their criticism of him “reprehensible” and helpful only to President Obama’s re-election.

Voters in Iowa have been swamped by nonstop television attack ads, robo-calls and mailers emanating from the campaigns and independent supporters of Republican candidates Mitt Romney, Ron Paul and Rick Perry. Among the claims contained in the ads: that Gingrich is a serial hypocrite and flip-flopper on issues such as a health insurance mandate, and that he made $1.6 million working for Freddie Mac, the mortgage giant blamed by many for the economic collapse.

“It’s candidly very disappointing to see some of my friends who are running put out so much negative junk,” Gingrich told scores of voters gathered at a security firm here.

Though he declined to criticize them by name and said he would continue to run a positive campaign, Gingrich’s disdain for his rivals was palpable.

“I really wish they would have the courage to be positive and I wish they would have the courage to have a campaign which would match ideas (rather) than see whose consultants can be the nastier,” he said.

Gingrich spoke as new polls showed him losing ground nationally and in Iowa, where the first contests will take place Jan. 3, as former Massachusetts Gov. Romney gains ground.

The former House speaker, who usurped Romney’s position as front-runner weeks ago, said he would hold telephone call-in town halls every day until the Iowa caucuses so voters can ask about the claims being leveled. He also said he would soon launch a 44-stop tour.

Gingrich’s newly announced moves are meant to prevent him from following the trajectories of Minnesota Rep. Bachmann, Texas Gov. Perry and businessman Herman Cain, all GOP candidates who in succession rose abruptly and fell nearly as swiftly. Only Romney and Texas Rep. Paul have had relatively stable, even increasing, support over the last few months.

The current slump is Gingrich’s second in several months. He lost almost all of his aides over the summer when, shortly after he entered the race, he took a two-week Mediterranean cruise instead of campaigning.

But as many GOP voters lost faith in the other candidates as alternatives to Romney, Gingrich was the most recent beneficiary. Yet the new Iowa polling shows a tight race, with Romney and Texas Rep. Ron Paul as front-runners and Gingrich in third.

“People are starting to remember why they didn’t think Gingrich was a strong candidate to start with,” said Tim Hagle, a political science professor at the University of Iowa.

Republicans rivals accused Gingrich of profiting from his former position as speaker and, in effect, lobbying members of Congress.

Gingrich said Freddie Mac hired his firm for six years, and that he personally received about $35,000 a year. He said he opposed any federal bailout of the organization.

As Gingrich tries to right himself in Iowa, his campaign also is under siege across the nation due to its disorganization.

Gingrich conceded that his campaign is struggling to gather enough signatures to get on the ballot in some states, with the next test occurring Thursday in Virginia. In the process, he took a veiled swipe at Romney.

“We’re scrambling. Look, there’s no question some candidates have been running for five or six years and have raised millions and millions of dollars and they’re better organized than I am,” he told reporters after the event. “On the other hand … I’m clearly one of the two front-runners, and we have a lot of popular support. The challenge for us is to get that popular support organized fast enough.”