So much for Ronald Reagan's supposed 11th commandment, "Thou shalt not speak ill of a fellow Republican."
Within 72 hours, Gingrich - who on the eve of the assault had been the front-runner for the GOP nomination in national polls - was transformed into an unelectable, mentally unstable risk.
First came wave after wave of big-time Republicans, testifying to the former House speaker's shortcomings in statements and calls organized by the Romney campaign. It was not hard to find former House colleagues ready to describe his "erratic" tenure in leadership, for instance, and veterans of the Reagan administration willing to undercut Gingrich's claims of closeness with the Gipper.
"Hardly anyone who served with Newt in Congress has endorsed him, and that speaks for itself," said former Kansas Sen. Bob Dole, the party's 1996 presidential nominee. "He was a one-man band who rarely took advice. It was his way or the highway."
Added Dole: "Gingrich had a new idea every minute, and most of them were off-the-wall."
Then there was the television blitz. Romney and his allies spent $15.4 million on ads in Florida, and only one of the spots those millions bought contained a positive message. (It was in Spanish and aired just 15 times statewide, according to Kantar Media's CMAG.)
Romney, with a new debate coach and an aggressive plan, also brought the heat in person.
In the Jan. 26 debate in Jacksonville, when Gingrich declined to defend his portrayals of Romney as living in a "Swiss bank account" world, the former Massachusetts governor hit back: "Wouldn't it be nice if people didn't make accusations somewhere else that they weren't willing to defend here?"
And he zeroed in on Gingrich's self-proclaimed title as the "big ideas" and "transformational" candidate, blasting his proposal to build a lunar colony by the end of his second term as the worst kind of pandering to idled Florida aerospace workers.
"I spent 25 years in business. If I had a business executive come to me and say they wanted to spend a few hundred billion dollars to put a colony on the moon, I'd say, 'You're fired,' " Romney said.
By the end of the Florida fight, Gingrich was reduced to complaining about the negative TV blitz, and Romney was openly mocking his foe - "Send him to the moon!"
"Speaker Gingrich, he's not feeling very excited these days," Romney said at a rally in Dunedin, practically snickering. "He's been flailing around a bit, trying to go after me for one thing or the other. You just watch and shake your head. It's been kind of painfully revealing to watch."
In a sense, Gingrich's challenge was the best thing that could have happened to Romney. He's a tougher, sharper candidate than he was before.
Of course, Romney still has his vulnerabilities, as he reminded everyone Thursday when he walked right into the "out of touch" rich-guy trap Democrats are laying for him, telling CNN he was not concerned about the "very poor." Never mind that the context was more complicated than that; it just sounds bad.
His education seems to have come at a cost, both from Gingrich's attacks and the blowback from going so relentlessly negative. In a Washington Post/ABC News poll in late January, 49 percent of respondents nationwide held an unfavorable view of Romney, while only 31 percent had a favorable one - a reversal from September.
Independents, in particular, have soured a bit on Romney. Favorable views of him among independents fell from a November high in the 40s to a low of 23 percent in the Post/ABC News survey. (Fifty-one percent of independents viewed President Obama favorably in the poll, up 7 points from December.)
But Romney, now running his second presidential campaign, knows candidates who don't survive the primaries don't have to worry about attracting independents in the fall.
Democrats may have figured the GOP field, especially Gingrich, was helping do their dirty work by pounding Romney. They might want to be careful what they wish for.
Contact politics writer Thomas Fitzgerald at 215-854-2718 or firstname.lastname@example.org or @tomfitzgerald on Twitter. Read his blog, "The Big Tent," at www.philly.com/BigTent.