Just moments after Newt Gingrich came under attack by two Mitt Romney surrogates, who pointedly characterized him as unpredictable, untrustworthy, and self-serving, the former speaker of the House responded in an interview with National Journal and CBS News, saying that he would take the high road in his race for the Republican presidential nomination. But he offered a warning to his fellow GOP candidates:

“My only observation,” he said, “is every time these guys have attacked each other, they’ve gone down. So I don’t know that being the attack dog in the Republican Party is necessarily an asset, you know a big asset. It’s not a game I’m going to play.”

After Gingrich’s rise to the top of several recent polls, overtaking Romney as the front-runner in the primary race, the Romney campaign stepped up its criticism of the former longtime lawmaker on Thursday morning. Two prominent Romney supporters, former New Hampshire Gov. John Sununu and former Sen. Jim Talent of Missouri, laid into Gingrich for a variety of perceived sins, from his attack on fellow Republican Rep. Paul Ryan’s Medicare plan to what they called “irrational” and “self-serving” behavior.

Gingrich declined to answer the attacks head on, instead saying he would stay focused on the “positive” -- a word he used eight times during the seven-minute interview. “I’m going to continue to run a positive, solution-oriented campaign. Others ought to do what they want,” he said. 

Asked if he would continue to remain above the fray if Romney himself attacks him, he replied, “If Mr. Romney goes on the attack, which is his prerogative – everybody should run the campaign that they’re comfortable with – my answers will be positive.”

Gingrich even praised Romney’s latest television ad, in which the former Massachusetts governor talks about being married for 42 years and attending the same church his entire life -- both perceived to be subtle digs at the thrice-married Gingrich, who recently converted to Catholicism.

“Callista and I like Mitt and Anne,” Gingrich said. “They have a good story to tell. He would be a fine nominee for this party and I would support him for the nominee of this party, so I think a positive ad that describes his life is terrific. We’ll do positive ads that describe us. People get to choose between the positives.”

In response to the claim by Romney surrogate Sununu that Gingrich had reneged on his approval to support a 1990 budget agreement that included tax increases after promising to support it, Gingrich said, “I think it’s fair to say that in my entire career I’ve been against tax increases. So it’s fair to say that I’ve had a remarkably consistent -- you know, some candidates zig-zag, other candidates are consistent. I’ve been very consistent.”

He said he had no idea why Romney’s surrogates were using the line of attack, adding, “I can’t imagine that Mitt favors tax increases, but you should ask them.”

Gingrich did call on Romney to release specific proposals in two areas: Medicare and immigration. Romney recently told the editorial board of the Washington Examiner that he has a “plan in mind” on illegal immigration.

“I mean, it would be nice – he has a semi-secret plan on Medicare, he has a secret plan on immigration. It would be nice for him to reveal them before the election,” Gingrich said.