He also said that his opponent's "Romneycare" Massachusetts health-care law called for state-funded abortions and that Romney had appointed liberal judges and raised taxes on business.
And Gingrich disputed the idea that Romney is the most electable candidate in the GOP field, arguing that three out of four Republicans rejected him in Iowa.
"Gov. Romney ran a largely negative campaign of falsehoods. ... The fact is, three out of four Republicans rejected it," Gingrich told reporters in Concord. "He'll do fairly well here. But the fact is, Gov. Romney in the end has a very limited appeal in a conservative party."
A new Suffolk University poll of New Hampshire released Wednesday showed Romney with 43 percent, Rep. Ron Paul of Texas with 14 percent, Gingrich with 9 percent, former Utah Gov. Jon Huntsman with 7 percent, and former Pennsylvania Sen. Rick Santorum with 6 percent. It was conducted Monday and Tuesday, before the Iowa results were known.
Gingrich effectively broke a pledge to run a positive campaign, although he didn't see it that way.
He contended that as long as he stuck to facts and accurately portrayed Romney's record - in recent days he has called Romney a liar in regard to Gingrich's own record - the strategy was not negative.
Gingrich was battered by a barrage of attack ads in Iowa that deflated his front-runner status. A number of them were run by a political action committee backing Romney.
Gingrich is seeking to reboot his White House bid. He is campaigning in New Hampshire, which holds its primary Tuesday, but has his eye on South Carolina, the first Southern primary, where evangelicals and social conservatives hold sway.
The pro-Gingrich super PAC Winning Our Future posted online a 2008 ad from John McCain's primary campaign highlighting Romney's flip-flops on abortion and gun rights and on being a Republican.
McCain endorsed Romney on Wednesday in New Hampshire. But in 2008, the two were rivals for the GOP nomination for president and the McCain camp pummeled him for swapping positions.
"Mitt Romney's flip-flops truly are masterpieces," the ad concludes.
Gingrich didn't congratulate Romney on Tuesday night in Iowa and declined an opportunity to do so Wednesday in New Hampshire.
"I know that's a rhetorical question," Gingrich told a reporter who had asked why he hadn't offered the traditional postelection congratulations to the victor.
"I suspect it's going to be a very lively campaign. I am delighted to be in New Hampshire to talk about big ideas, big solutions, and a big contrast," he said.
This article contains information from McClatchy Newspapers.