Daniel Acker for The New York Times

In Iowa Reporting on candidates and voters from the Hawkeye State.

DES MOINES — Weeks of staying cheerily — and ineffectively — positive amid an onslaught of negativity appears to have finally gotten to Newt Gingrich.

Tired, frustrated and irritated, by his own account, Mr. Gingrich on Tuesday morning blasted Mr. Romney, calling him a liar and accusing him of misstating both his own record and the records of his rivals.

Asked directly whether he was calling Mr. Romney a liar, Mr. Gingrich said, “Yes.”

When asked the question again, he continued: “Well, you seemed shocked by it. Yes.”

“He’s not telling the American people the truth. It’s just like his pretense that he’s a conservative,” Mr. Gingrich said on CBS’s “The Early Show.” “I just think he ought to be honest to the American people and try to win as the real Mitt Romney, not try to invent a poll-driven, consultant-guided version that goes around with talking points.”

The volume of Mr Gingrich’s complaining has gotten louder and louder each day.

At a stop in Marshalltown, Iowa, on Monday, Mr. Gingrich complained that he has been “Romney-boated,” a reference to the Swift Boat Veterans for Truth ads that helped derail Senator John Kerry’s presidential campaign in 2004.

Appearing on CNN’s “Piers Morgan Tonight” on Monday, Mr. Gingrich repeated his complaints about Mr. Romney’s refusal to disown the negative ads being run by a “super PAC” supporting his campaign.

“If he would be willing to just be man enough to say, ‘You know, this is my negative campaign and I admit it,’ I’d be a lot happier,” Mr. Gingrich said. “What I find very frustrating, and frankly irritating,” he said, is someone who “wants to run for president of the United States who can’t be honest with the American people.”

Mr. Gingrich and his aides have said that the end of the Iowa caucus will bring a shift in his approach to the campaign. Gone will be the gentle, positive Mr. Gingrich. In its place, they say, will be an aggressive assault on Mr. Romney’s record.

Mr. Romney responded to that possibility in an interview Tuesday morning on Fox’s “Fox and Friends” program.

“Well, I understand Newt must be very angry and I don’t exactly understand why, but look, I wish him well,” Mr. Romney said. “It’s a long road ahead. He’s a good guy. I like he and Callista. We’ve got many months ahead of us, so I’ll leave it at that.”

It could be too late. Polls suggest that Mr. Gingrich has fallen from his once-lofty place atop the Republican field into the second tier of candidates who are struggling for a fourth-place finish so they can make a strong case to stay in the race.

Mr. Gingrich and his aides are clearly strategizing — hoping to make that case however the caucus turns out tonight.

But in the meantime, it’s definitely getting under his skin.

“Here is my simple tag line: Somebody who will lie to you to get to be president will lie to you when they are president,” he said at the appearance in Marshalltown.