Romney recently released an ad showing an archival clip of NBC anchor Tom Brokaw delivering the news in 1997 that Gingrich's peers in the House "found him guilty of ethics violations," charged him a "very large financial penalty" and "raised serious questions" about his future as House speaker.
"It's fundamentally false," Gingrich told me Sunday on "This Week." "It's typical of [Romney's] whole campaign. He knows… that this is a purely phony charge."
Gingrich noted that he was "attacked" with 84 allegations of ethics violations by House Democrats after he helped usher in the first Republican take-over of the lower chamber in 40 years.
"Every single one of the substantive charges was ultimately thrown out, period," Gingrich said, referencing a 1999 IRS report that found Gingrich had not violated any tax laws.
The Romney ad is part of a more than $8 million ad war that Romney and his supporting Super PAC have launched against Gingrich in Florida.
The former House speaker said a deluge of negative ads such as this is something the eventual GOP nominee is going to have to be able to weather in the general election.
"Look, whoever gets to be the Republican nominee is going to be attacked by Obama's billion-dollar attack campaign," Gingrich said. "The question is, can you withstand that attack better as a solid conservative or as a moderate?"
Reiterating a line from his own attack ads, Gingrich said Romney, a "Massachusetts moderate," is "very unlikely" to hold up against such an attack in the general election and just as likely to crumble under his "liberal record" in the primary.
"I can assure you that all the way to the convention we're going to have a fight over whether or not somebody can be fundamentally dishonest and try to hide their liberal record in Massachusetts and try to hide their past, in terms of voting for Democrats, and get to be the nominee," Gingrich said.