Former Massachusetts governor Mitt Romney’s campaign opened up a new line of attack against Newt Gingrich this morning, calling into question the former House Speaker’s qualifications to be commander in chief.

“The off-the-cuff comment, for example, that Gingrich throws out on occasion is a reflection on the off-the-cuff thinking that he goes through to deal with issues, and that is not what you want in a commander in chief,” said former New Hampshire governor and White House chief of staff John Sununu, a Romney supporter. “What he did to [Wisconsin Rep.] Paul Ryan is a perfect example of the irrational behavior that you do not want in a commander in chief.”

Former House speaker Newt Gingrich and former Massachusetts governor Mitt Romney cross paths during a break in the CNN GOP National Security debate in Washington last month. (REUTERS/Jonathan Ernst)

Sununu’s comments came on a conference call aimed at Gingrich’s criticism of Ryan’s budget proposal and not foreign policy. By questioning Gingrich’s ability to be commander in chief, the Romney team is launching a major attack that could have reverberations not just in the primary fight but also in the general election if Gingrich winds up as the nominee.

Going after a candidate’s personal qualities and issue positions is one thing; going after his or her ability to defend the United States is quite another.

You may remember the furor in the 2008 Democratic presidential campaign when then New York Sen. Hillary Clinton’s campaign came out with its famous “3 a.m.” ad, which some saw as a not-so-subtle attempt to suggest then Illinois Sen. Barack Obama wasn’t ready to be commander in chief.

Having a surrogate launch such attacks isn’t as powerful as running an ad to that effect, and Sununu’s comments didn’t seem to be planned — his “off the cuff” remarks appeared to be genuinely off the cuff — but they still represents a pretty significant attack in a race that hasn’t seen all that much back and forth to date.

Romney will now be asked whether he agrees with Sununu that Gingrich doesn’t have the necessary restraint to be commander in chief and he’ll need to decide whether he backs up his surrogate or seeks to downplay the attack.

Regardless, Sununu’s comments represent a powerful opening salvo against Gingrich, and, if conservatives see the Romney campaign’s attacks on Gingrich as beyond the pale, there could be a rallying effect behind the former House Speaker.

At this point, of course, Romney’s team needs to try something to regain its position as the frontrunner in the race.

The fact that Romney’s campaign is moving so forcefully to call into question Gingrich’s presidential bona fides shows just how real they see him as a threat to Romney’s path to presidential victory. CNN/Time polls released in the four earliest states Wednesday showed Gingrich leading by double digits in three of the four states and gaining in New Hampshire.