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 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 quasigovernment 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 in an interview from the Chez Vachon diner, a regular New Hampshire stop for presidential candidates.

Romney suggested that Gingrich was an ultimate insider who 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 quickly, saying Romney should give back the millions he made working at Bain Capital Partners L.L.C., 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," Gingrich 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.

Unbowed, Romney chided Gingrich anew: "There's a big difference between working in the private economy and working on K Street, and working as a lobbyist and working as a legislator, and working to connect businesses with government."

Romney's stepped-up criticism came two days after Gingrich held his own during Saturday's debate in Iowa, turning aside attacks from Romney and other rivals. In recent weeks, Gingrich has risen to the top of polls nationally and in early-voting states. He has even started to eat into Romney's lead in New Hampshire, a must-win state for the former Massachusetts governor.

As the first voting nears - Iowa's caucuses are Jan. 3, and New Hampshire's primary is Jan. 10 - Romney has been promising a more aggressive campaign style.

"We aren't running any negative ads ... but we may," Romney said Monday.

Romney's allies are already running negative ads in Iowa assailing Gingrich's record in Washington. Also piling on is Rep. Ron Paul of Texas. He has surged into the top three in Iowa polls. He released a Web ad Monday 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 he would stay faithful to his wife. Gingrich has been married three times and has acknowledged a previous extramarital affair.

Also Monday, Gingrich and GOP competitor Jon Huntsman held a no-rules debate that Gingrich said he was thrilled with. Their free-flowing foreign-policy discussion at New Hampshire's St. Anselm College was modeled after the 1858 debates between Abraham Lincoln and Stephen Douglas.

At the 90-minute event, which had no strict time limits or formal moderator, Gingrich and Huntsman agreed on virtually everything. But Gingrich said more substantive debates were needed.