Republican presidential candidates former Massachusetts Gov. Mitt Romney, left, and former U.S. House Speaker Newt Gingrich attend the Iowa debate. (JEFF HAYNES/REUTERS)

Former Massachusetts governor Mitt Romney escalated his rivalry with Newt Gingrich on Monday with a series of pointed, personal attacks, signaling a more aggressive and negative shift in the race for the Republican presidential nomination.

Romney took his first jab in an early television interview in which he called on Gingrich to return the more than $1.6 million he earned from consulting for Freddie Mac — the start of a back-and-forth over which candidate is greedier.

Gingrich has said he served as a historian for the mortgage giant, to which Romney remarked: “That would make him the highest-paid historian in history.” Romney added, “One of the things that I think people recognize in Washington is that people go there to serve the people and then they stay there to serve themselves.”

Gingrich responded at a morning event in Londonderry, saying he would consider returning his Freddie Mac earnings “if Governor Romney would like to give back all the money he’s earned bankrupting companies and laying off employees over his years” — a reference to Romney’s time as head of the private equity firm Bain Capital.

“But I bet you $10 — not $10,000 — that he won’t take the offer,” Gingrich added in a reference to Romney’s widely mocked offer to bet Texas Gov. Rick Perry at a candidates debate Saturday night. Gingrich appeared to ease off his attacks later in the day, however, thanking Romney at a nighttime rally for saying that he didn’t want to weaken any of his opponents.

Nonetheless, the exchanges between Romney and Gingrich marked a continuation of the battle lines that emerged during the debate and are likely to solidify over the next three weeks, when voters start to have their say. Romney’s decision to go after Gingrich in New Hampshire, where he has invested heavily and has been favored to win all year, underscores how crucial the state is to his prospects for winning the GOP nomination — and how worried he is about Gingrich’s rise in the polls.

After Gingrich’s comments about his time at Bain, Romney sought to cast the former House speaker as a Washington insider during an event that drew about 100 supporters to a lumber mill in Madison.

“There’s a big difference between working in the private economy and working on K Street and working as a lobbyist or working as a legislator or working to connect businesses with government,” Romney said. He also laid into Gingrich for saying in the spring that House Budget Committee Chairman Paul Ryan’s plan to overhaul entitlements was ”right-wing social engineering.”

Later in the day, Gingrich reiterated his promise to remain focused on ideas and issues, a strategy meant to leave Romney looking defensive and desperate as he seeks to catch up to his latest chief rival. The former speaker also sought to emphasize his high-road strategy at a joint appearance with former Utah governor Jon Huntsman Jr. that was notable mostly for the amount of praise the pair lavished on each other.

At a nighttime event in Windham, Gingrich made a more direct appeal for a cease-fire among the Republican candidates, drawing enormous applause from a crowd of about 1,000.

Romney “said it’s very important that we not weaken any of the people who might defeat Barack Obama and he would rather lose than engage in outrageous attacks against one of his competitors, and I want to thank him for taking that position,” Gingrich said, quoting remarks Romney made in an interview with Politico.

At his appearance with Huntsman at St. Anselm College, Gingrich said Huntsman had done a “tremendous job” as Utah governor. Huntsman called Gingrich a “great historian.”

“This is how it could be,” Gingrich said at the close of the two-hour event — a reference to the tenor of Romney’s attacks. Afterward, Huntsman praised the former speaker for “an excellent performance.”

Gingrich and Huntsman share a mutual interest. If both do well in next month’s first-in-the-nation primary in New Hampshire, they have the potential to deal Romney a severe blow by denying him a clear victory.

Combined, Gingrich and Huntsman could cut deeply into Romney’s margin, because they appeal to different constituencies within the electorate. Gingrich is likely to run well among conservatives. Huntsman — even though he has not made a strong showing in the polls — is hoping to tap support from moderates as well as independents. New Hampshire’s is an open primary, which means a large share of the electorate is likely to be independent voters.

If Romney “doesn’t win New Hampshire by double digits, it’s not going to look good,” said state Rep. Jack B. Flanagan, a Romney supporter.

Under the “Lincoln-Douglas” format of the Gingrich-Huntsman debate, the moderator posed topics, not questions, which left the contenders great leeway to frame their presentations and to go deep, with some answers lasting five minutes — and some members of the audience drifting off to sleep.

“I can see my daughter nodding off over there,” Huntsman quipped at one point.

Asked about Iran, Gingrich once again said that regime change would be necessary to ensure that the country does not develop nuclear weapons.

“I agree with a lot of what the speaker has put forward on Iran,” Huntsman said, adding that he lamented what he said had been a “lost opportunity” to support Iranian dissidents last year.

They also did not rule out the possibility of backing Israel should it strike Iran to hurt its nuclear capability.

“The mullahs in Tehran need to know that all options are on the table,” Huntsman said.

“I agree with Governor Huntsman,” Gingrich replied, adding, “This is not a very far-down-the-road decision.”

At one point, Huntsman even broke into Mandarin, recalling that Chinese officials used to tell him: “In China, we have politics, too.”