The answer seems obvious, right? After all, Mr. Gingrich – author of the Contract With America and architect of a new conservative movement when he was in the House – has been embraced largely by GOP voters worried that Mr. Romney isn't far removed from being a Democrat.
Not if Romney has his way. The former Massachusetts governor is stepping up his efforts to slap Gingrich with the "moderate" label, highlighting his willingness to compromise on certain issues in the past and trying to spread doubt among GOP conservatives as to whether Gingrich is really the man they want.
"Pelosi and Gingrich supported Al Gore's liberal global warming agenda," the ad says, showing three-year-old footage of an ad Gingrich and Pelosi shot together in favor of the initiative. The two are sitting on a couch and smiling together, and Gingrich declares, "We do agree, our country must take action to address climate change."
"With friends like Newt," the ad concludes, "who needs the left?"
Romney also took this tack in an interview with the Washington Post published Tuesday, in which he hammered Gingrich for the climate-change ad as well as Gingrich's criticism of Paul Ryan's Medicare reform proposal – which Gingrich called "right-wing social engineering" at the time.
“He has been an extraordinarily unreliable leader in the conservative world – not 16 or 17 years ago but in the last two to three years,” Romney told the Post. “And even during the campaign, the number of times he has moved from one spot to another has been remarkable. I think he’s shown a level of unreliability as a conservative leader today.”
He got some help Wednesday morning from tea party favorite Christine O'Donnell, who also told CNN that Gingrich is "inconsistent and unreliable" and offered her endorsement to Romney (who, she said, has "been consistent since he changed his mind.")
Of course, Romney's attempts to paint Gingrich as a moderate – or at least as an "unreliable" conservative – stem largely from his own vulnerabilities. He's been repeatedly called out by his opponents, in particular, for creating a health-care plan in Massachusetts that may have served as a template for Obama's national health-care plan.
And simply serving as governor of Massachusetts is enough to taint him in many Republicans' eye – a fact he acknowledged in the Washington Post interview. (He also said that he has grown more conservative over time, that he now regrets not signing Gingrich's Contract With America, and that his "admiration and respect for the policies of Ronald Reagan has grown.")
Indeed, Gingrich hasn't had to attack Romney very directly for his lack of conservative credentials since everyone else is doing it for him.
So, what could be the result if all the negativity works, and conservative Republicans are left without a solid candidate to rally behind? One long-shot possibility, outlined by New York Times polling analyst Nate Silver: a victory by a more consistent moderate, Jon Huntsman.
While that outcome seems unlikely, its chances increase, says Silver, if Gingrich does just enough damage to Romney to keep him from the nomination, while also failing to convince voters himself.
As usual, look for the rhetoric from whoever gets the nomination to shift markedly in the general election, when "moderate" may not be quite the dirty word it is now.