In the increasingly acerbic nomination fight, Romney fired back at Gingrich during a morning debate: "This ain't beanbag ... we're going to describe the differences between us." By evening, he also had taken shots from Rick Santorum, Jon Huntsman and Rick Perry.
With time running short to curtail Romney here and perhaps elsewhere, his opponents started the day assailing him on the debate stage and ended it by doing the same in appearances across New Hampshire and South Carolina as they worked to appeal to the chunk of Republican voters unenthused with the idea of the former Massachusetts governor as the party's nominee.
Santorum made a beeline to the conservative upstate of South Carolina to trumpet the endorsement of former Republican presidential candidate Gary Bauer.
"I've still got a little blood on my sleeve from Mitt Romney from that debate," the former Pennsylvania senator told 400 people crammed into Chief's sports bar in Greenville, S.C. "We're not going to shy away where there are differences." He also alluded to Romney's position switches on a series of issues, saying: "We've got a lot of candidates that just adapt to whatever the environment is .... I don't, because the truth doesn't change."
Looking to revive his flagging candidacy, Perry also swooped into the Southern state.
The Texas governor told roughly 300 people at a packed burger joint in Spartanburg, S.C., that his campaign, after a disappointing finish in the Iowa caucuses, was like the last stand at the Alamo. He also assailed Romney, casting him as an insider and arguing: "We've got to have somebody that is an outsider that is not interested in tinkering around the edges - but that will go into Washington, D.C., and overhaul that place."
Back in New Hampshire, Gingrich assailed Romney as a "Massachusetts moderate" and promoted a video being released by his allies that attacks Romney's business career. The Gingrich-leaning Winning Our Future PAC said Sunday that a 28-minute online video - which assails Romney for "reaping massive awards" while head of Bain Capital - may show up on TV in the coming weeks.
"To quote the governor, you have to have broad shoulders and you have to be able to take the heat to be in the kitchen," Gingrich said after an afternoon town-hall style appearance at Manchester restaurant.
Romney, the former Massachusetts governor, won the Iowa caucuses last Tuesday by a scant eight votes over Santorum but is so far ahead in New Hampshire polls that his rivals have virtually conceded he will win.
South Carolina comes next, on Jan. 21, the first Southern state to hold a primary. While it is the contest where Gingrich, Santorum and the rest of Romney's rivals face an urgent need to slow his candidacy, Romney pointedly noted that he has been endorsed by that state's governor, Nikki Haley.
Santorum finished second in Iowa, followed by Texas Rep. Ron Paul, with Gingrich fourth, Texas Gov. Rick Perry fifth and Minnesota Rep. Michele Bachmann in last place. She has since quit the race.
Huntsman, the former Utah governor, skipped Iowa in hopes of a breakout showing in New Hampshire.
He was mobbed at a coffee shop in Hampstead, where he stood on the counter to defend his past service in the Obama administration and assail Romney, saying: "I put my country first. Apparently Mitt Romney doesn't believe in putting country first. He's got this bumper sticker that says ... Believe in America. How can you believe in America when you're not willing to serve America? That's just phony nonsense."
Sunday began with GOP contenders facing off for the second time in less than 12 hours, following their debate Saturday night in nearby Manchester.
Though aggressive in assailing Romney, Gingrich hedged when he was confronted with one of his own campaign leaflets declaring Romney to be unelectable against President Barack Obama. "I think he'll have a very hard time getting elected," was as far as Gingrich would go.
Indicating he'd taken offense on another matter, Huntsman, who was Obama's ambassador to China before quitting to run for the White House, returned to a comment Romney had made the night before. Romney said then that the rest of the GOP hopefuls had been trying to oppose the administration's policies while Huntsman was advancing them.
"And I just want to remind the people here in New Hampshire and throughout the United States, he criticized me while he was out raising money for serving my country in China, yes, under a Democrat, like my two sons are doing in the United States Navy," Huntsman said Sunday. "They're not asking what political affiliation the president is."
As was the case Saturday night, Romney sought to shrug off the attacks from his rivals on the debate stage and worked to turn the focus onto Obama.
But Gingrich was more aggressive Sunday morning than he had been Saturday night, his attacks serving as bookends to the 90-minute event.
The former speaker briefly led in the Iowa and national polls before the caucuses, before his surge was blunted by a series of ads aired by a so-called super PAC that is operated by former Romney aides and allies.
Gingrich has complained bitterly that the attacks were false, but he was asked Sunday about a similar organization set up by his own supporters. It is intent on criticizing Romney for having run an investment firm that cost workers their jobs when it took over their companies.
Asked if he was being consistent, Gingrich said, "I'm consistent because I think you ought to have fact-based campaigns." He demanded Romney say whether the attacks against him were true.
Romney replied: "I haven't seen them, and as you know, under the law, I can't direct the ads. If there's anything in the ads that are wrong, I hope they take it out."
Yet moments after saying he hadn't seen the commercials, he recited the charges they made and said they were accurate - that Gingrich had been forced to resign as speaker, that he had once talked of finding common ground with House Democrats on climate change and that he had called a House Republican proposal to overhaul Medicare "right-wing social engineering."
Gingrich said he was glad Romney "has said weeks later if they're wrong they should take them down."
Santorum pointedly asked Romney during the debate why he hadn't sought re-election after one term as governor in the neighboring state.
"Why did you bail out?" Santorum asked.
Romney fired back with a reference to Santorum's lucrative career in the six years since he lost his Senate seat. Describing politicians who lose office but stay in Washington "and make money as lobbyists or conducting their businesses," Romney said, "I think it stinks."
Moments later, Gingrich appeared irked and accused Romney of using more than his allotted time to respond: "I realize the red light doesn't mean anything to you because you're the front-runner."
Gingrich also dismissed Romney's claims to being a political outsider: "Could we drop a little bit of the pious baloney? The fact is you ran in '94 and lost (to Massachusetts Sen. Edward Kennedy). ... You were running for president while you were governor. ... You've been running consistently for years."
Associated Press writers Holly Ramer in New Hampshire and Thomas Beaumont and Jim Davenport in South Carolina contributed to this report.