MANCHESTER, N.H. – Trading accusations of greed, Mitt Romney and Newt Gingrich challenged each other Monday to return millions made in private business as the race for the GOP presidential nomination turned increasingly acerbic and personal at the start of a three-week sprint to the Iowa caucuses.

Far from Iowa, the two men campaigned miles apart from each other in next-up New Hampshire, where Romney has long dominated in polls but where Gingrich is aggressively working to make inroads.

Romney called on Gingrich to return the estimated $1.6 million he received for providing strategic advice to Freddie Mac, the quasi-government agency that guarantees home mortgages. Gingrich has said he acted as a historian, not a lobbyist.

“That would make him the highest paid historian in history,” Romney told Fox News Channel during an interview from the Chez Vachon diner, a regular New Hampshire stop for presidential candidates.

He suggested that Gingrich leveraged his position as a former House speaker to line his pockets when he left office. Said Romney: “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, campaigning in nearby Londonderry, countered that Romney should give back the millions he made working at Bain Capital, a venture capital firm that sometimes laid people off as part of its efforts to make businesses more efficient.

“If Gov. Romney would give back all the money he’s earned from bankrupting companies and laying off employees over the years at Bain, then I would be glad to listen to him,” Ging-rich said. “But I bet you $10 — not $10,000 — that he won’t take the offer.”

That was a dig at Romney’s offer of a $10,000 wager with Rick Perry at Saturday night’s debate.

In recent weeks, Gingrich has risen to the top of polls nationally and in early voting states. He’s even started to eat into Romney’s lead in New Hampshire.

Time is running short for Romney to curb Gingrich’s rise, with the caucuses in Iowa on Jan. 3, and the New Hampshire primary on Jan. 10 .

Romney has been promising a more aggressive campaign style.

“We aren’t running any negative ads but we may,” Romney said Monday. “This is, after all, politics. There’s no whining in politics.”

Romney’s allies already are running a negative ad campaign in Iowa assailing Gingrich’s record in Washington. Also piling on is Texas Rep. Ron Paul, who has surged into the top three in Iowa polls. Paul on Monday released a Web ad accusing Gingrich of “selling access” in Washington and including a clip of Gingrich calling himself an insider.

Gingrich spent part of the day working to alleviate concerns about some of his personal baggage that could hurt him with social conservatives.

He sent a letter to the Family Leader, a key group in Iowa, pledging that he would stay faithful to his wife. Gingrich has been married three times and has acknowledged a previous extramarital affair.

Gingrich said he’s the front-runner in national polls and noted that he’s leading in South Carolina and Florida. He told his audience at Insight Technology, a Londonderry military contractor: “I’m behind a little bit here, so I need your help to finish off here and win here.”