DAVENPORT, Iowa — Facing slipping poll numbers, an angry Newt Gingrich lashed out at his GOP presidential rivals on Monday, calling their criticism of him "reprehensible" and helpful only to President Barack 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 in the GOP 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, 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.