Romney, who is a former governor in neighboring Massachusetts, had been holding a commanding lead in the state, but Gingrich is edging in on him.
New Hampshire's key newspaper, The Union Leader, endorsed Gingrich over Romney a few weeks ago, which was a blow to the former governor, who was passed over for John McCain by the paper in 2008.
Monday morning Manchester's mayor Ted Gatsas will endorse Romney.
Sunday Gatsas said, "[Romney] has the skills and the executive experience that the next president will need to turn around our struggling economy" and that he's "impressed with the time and effort that [Gov. Romney] has invested in New Hampshire."
New Hampshire holds the first-in the-nation primary on January 10, exactly one week after the Iowa caucuses, which are on January 3.
The Romney campaign has pumped up its attacks against Gingrich, former U.S. Speaker of the House who has soared in recent polls.
The new battle between the GOP frontrunners is one that President Obama even says will go on for awhile, when asked about Gingrich's recent surge.
"He's somebody who's been around a long time, and is good on TV, is good in debates. And, you know, but Mitt Romney has shown himself to be somebody who's good at politics, as well. He's had a lot of practice at it. You know, I think that they will be going at it for a while," Obama said in an interview on CBS' "60 Minutes."
Gingrich, who has become known for his good debate performances, takes on another GOP hopeful, Jon Huntsman, in a Lincoln-Douglas style face-off at St. Anselm College late Monday afternoon.
Huntsman has had a one-state strategy, trying to lock in New Hampshire, avoiding campaigning in Iowa all together.
Continuing to struggle in the polls, he did not take part in Saturday's Iowa debate, opting to campaign in the Granite State instead.
Despite his polling, he still think there is time and not everyone has started paying attention yet.
He said on ABC's "This Week" that "the marketplace is still open, people are shopping" who they'll vote for that notes that New Hampshire likes to go against the trend so he's still got a shot to do well.