Gingrich called Romney a "Massachusetts moderate who, in fact, is pretty good at managing the decay." He said the former governor has "given no evidence in his years in Massachusetts of any ability to change the culture or change the political structure."
Gingrich's speech Tuesday night signaled that he would continue to take the fight to Romney, whom he has criticized for running a negative campaign. Gingrich also said he does not foresee the GOP race ending anytime soon.
"One of the things that became obvious in the last few weeks in Iowa is that there will be a great debate in the Republican Party before we are ready to have a great debate with Barack Obama," the former House speaker said.
Gingrich congratulated two other contenders, Rick Santorum and Ron Paul, for strong showings but emphasized he will air contrasts between himself and Romney. He praised Santorum - who was running neck and neck with Romney to win the caucuses - for running a "great, positive" campaign.
Earlier Tuesday, Gingrich vowed to fight on, blaming his disappointing showing in Iowa on attack ads against him.
Gingrich told The Associated Press while grabbing a snack at his hotel coffee shop that he just needed a little more time to reach out to voters and rebuild the lead he once held but that evaporated in a barrage of attack ads.
"I think we did OK," Gingrich said, acknowledging that he ended up in roughly the same place he'd been before his surge. He said the negative ads had taken a toll.
"We needed two more weeks," he said.
Gingrich plans to continue to hit back at Romney as the campaign moves to New Hampshire on Wednesday. Gingrich said he's putting up television ads in that state along with South Carolina and Florida that aggressively contrast his record with Romney's.
Gingrich watched his surge in Iowa polls evaporate amid a barrage of attack ads, many run by a political action committee backing Romney.
Gingrich's first planned salvo was a full-page ad in the New Hampshire Union Leader that was to appear as he touched down in the state early Wednesday. It labels Gingrich a "Bold Reagan Conservative" and Romney a "Timid Massachusetts Moderate."
At least one pro-Gingrich super PAC also was getting into the mix.
"We definitely plan to engage," said Rick Tyler, a former longtime Gingrich aide now working for the pro-Gingrich super PAC Winning our Future. "I think that Newt has signaled a communications shift in which he wants to define Romney and to draw a contrast between his record and Romney's."
Tyler declined to be specific about the group's plans. The independent political action committees are banned by law from coordinating with the candidates they are supporting.
Spending by pro-Gingrich groups in Iowa was anemic compared to the millions of dollars flowing from PACs supporting Romney and Texas Gov. Rick Perry, according to federal campaign finance data.
A key question is whether Gingrich will have the cash to wage an effective assault. A spokesman said he'd raised roughly $9 million in the last three months of the year but that he would spend most of it to compete in Iowa. It is also unclear how much money the pro-Gingrich PACs have raised.
Going on the offensive against Romney also places Gingrich in a tricky position: He has been bashing super PAC spending by Romney and saying it's dishonest to hide behind independent groups doing a candidate's dirty work. And he's promised to wage a positive campaign, even as earlier Tuesday he painted Romney as a liar.
Gingrich said it is not negative "to accurately describe someone's record."
"He doesn't tell the truth," Gingrich said.
The former Georgia congressman previewed his likely lines of attacks in Burlington, Iowa, as he ticked off what he said was Romney's past support for gun control and the inclusion Planned Parenthood in the Massachusetts health law he signed as governor.
Still, Gingrich insisted his ads would not be negative.
"All we have to say in a happy and positive way is 'Newt believes in the Second Amendment, here's what Romney said about guns. Newt believes in right to life and here's what Romney did with Romneycare,'" he said. "You can do that pretty happily and have happy music."
For his part, Romney said Tuesday in Iowa that he expected to face an onslaught.
"It's a long road . I expect people to come after me," he said in an interview on MSNBC. "And if I do well here, I'll have a target painted on me, and so I expect other folks to come after me. ... And, you know, if I can't stand up to that, I shouldn't be the nominee."