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Gingrich’s undercutting of Paul Ryan proves that he is more concerned about Newt Gingrich than he is about conservative principles,” Mr. Sununu said, alluding to Mr. Gingrich’s characterization of the plan as too “radical” and “right-wing social engineering.”

Mr. Talent said that reaction to the Ryan plan is an example of how he often says “outrageous things that come from nowhere.”

“He has a tendency to say them at exactly the time when they most undermine the conservative agenda,” he said.

Others said the former speaker was getting a raw deal. Former Rep. Christopher H. Shays of Connecticut told The Washington Times that some of the criticism leveled at Mr. Gingrich was “over the edge.”

Without Mr. Gingrich, he said, Congress wouldn’t have passed welfare reform or balanced the federal budget.

“The only two people I know who had significant impact on Washington in the sense of changing it are Ronald Reagan and Newt Gingrich. He got a plane that hadn’t flown in 40 years to fly, and nobody thought it could,” Mr. Shays said, alluding to the lead role Mr. Gingrich played in 1994 in pushing the Republicans’ “Contract With America” and helping Republicans take control of the House for the first time in four decades.

Despite his defense of Mr. Gingrich, Mr. Shays has endorsed Mr. Romney and said he is better positioned to lead the country, putting him in line with other Republicans who served under Mr. Gingrich and applaud his smarts, but haven’t endorsed him.

Former Rep. Michael N. Castle of Delaware suggested that some of Mr. Gingrich’s former colleagues are wary of him because a lot of the ideas he touted never came to fruition.

“Newt is extremely bright and has a lot of ideas — a number of which are very sound about how to make government more efficient,” he said. “Unfortunately, those ideas didn’t always translate into the accomplishment of getting something done or the refining of whatever his idea might be to the point where a majority could support it.”

• Dave Boyer contributed to this report.