DES MOINES, Iowa - In a scathing attack on former House Speaker Newt Gingrich's record, two surrogates for Mitt Romney today described the GOP front-runner as an unreliable and untrustworthy conservative who makes "self-aggrandizing comments" in an attempt to make himself "sound a little smarter." 

Former New Hampshire Gov. John Sununu and former Missouri Sen. Jim Talent unleashed the most outright criticism of Gingrich by the Romney camp yet in a news conference call this morning to contrast Romney and Gingrich's views on Rep. Paul Ryan's Medicare plan. They honed in on Gingrich's comments in May when he referred to the plan as a "radical change" as an example of the former speaker's penchant for making off-the-cuff remarks.

"The speaker is running as a reliable and trusted conservative leader and what we're here today to say with reluctance he's not a reliable and trusted conservative," Talent said bluntly.

Adding again and again that he was "reluctant" to offer such harsh criticism of a man who was once his own leader in the House, Talent said, "Speaker Gingrich says interesting and insightful things, and he can explain them well on many occasions. He also says outrageous things and he had a tendency to say them when they most undermine the conservative agenda."

Referring to Gingrich's comments on the Ryan plan, which the former speaker also quipped was an example of "right-wing social engineering," Talent said the remarks "came from nowhere" and that Ryan was "blindsided."

Gingrich later called Ryan to apologize for his comments, telling Fox News after he did, "I made a mistake and I called Paul Ryan today. He's a very close personal friend, and I said to him, the fact is that I have supported what Ryan's trying to do on the budget.

"The budget vote is one that I am happy to say I would have voted for. I will defend, and I'd be glad to answer any Democrat who attempts to distort what I said," Gingrich said.

But Sununu today didn't let go of Gingrich's original comments. "For Newt Gingrich, in an effort of self -aggrandizement, to come out and throw a clever phrase that had no other purpose than to try to make him sound a little smarter than the conservative Republican leadership, to undercut Paul Ryan, is the most self-serving, anti-conservative thing one could imagine happening," Sununu said.

"He gave the liberals and the Dems the ammunition they needed to mute, at least for the time being, Paul Ryan's presentation. The off-the-cuff comments, for example, that Gingrich throws out is a reflection of the off-the-cuff thinking that he goes through to deal with issues," Sununu said. "And that's not what you want in a commander and chief."

As for Romney's support of the Ryan plan, the former Massachusetts' governor responded "yes" when asked if he would sign the plan written by Ryan that would restructure Medicare if it reached his desk as president, but quickly added that he would be offering his own plan.

"His plan is not the plan I'll put forward, I have my own plan," Romney said. "I'll be putting that out before I debate President Obama."

Romney has since introduced his own plan for entitlements, proposing a "premium support" account for beneficiaries that would allow them to make a choice between private plans and Medicare.

Asked about the Romney campaign's comments, Gingrich responded to reporters in South Carolina: "I'm going to stay positive, I'm going to talk about how we solve the country's problems. And I have one opponent, Barack Obama, and that's how our campaign is going to keep moving forward. Others are allowed to do what they want to do."

As for whether the back and forth was getting too personal, Gingrich said, "No, he's fine."

ABC News' Elicia Dover contributed to this report.