Efforts to curb Mitt Romney's momentum in the Republican presidential race escalated on Monday when he became the target of a scathing ad campaign.

A group supporting rival candidate Newt Gingrich has taken aim at the frontrunner with biting ads and an online film trailer depicting Romney as a "corporate raider."

The campaign by Winning our Future, a political action committee (PAC), isn't authorized by Gingrich but intends to attack his rival's history as head of private equity firm Bain Capital.

"His mission? To reap massive rewards for himself and his investors," a narrator says in a trailer for "King of Bain: When Mitt Romney Came to Town."

The film goes on to accuse him of taking "seed" money from Latin America and "exploiting dozens of American businesses."

It's perhaps one of the most piercing attacks in a race where the former Massachusetts governor has yet to be dethroned as lead candidate.

The pro-Gingrich PAC received a $5 million leg up from a wealthy casino owner to produce the project, The New York Times reported on Sunday.

Romney's multibillion-dollar venture capital firm has been mentioned by rivals before, but as GOP hopefuls try to capture New Hampshire, detractors have ramped up their attempts to stop his lead.

With only a day to go until New Hampshire's primary, Republican presidential hopefuls are hitting the campaign trail hard in a last-ditch attempt to woo voters.

Candidates including Romney, Gingrich and Jon Huntsman fanned out across the crucial battleground on Monday, trying to reach large audiences at multiple campaign stops.

Washington's WTOP radio political commentator Mark Plotkin says the focus for most remaining GOP hopefuls is trying to stop Romney's momentum.

He said the former Massachusetts governor, who holds a significant lead in pre-primary polls, continues to pose a threat to others vying for the party's top job.

"He's the only candidate right now who can really run a national campaign," Plotkin told CTV's Canada AM on Monday.

Romney, who has spent about $1 million on television ads, won last week's nail-biting Iowa caucuses with only eight votes over Santorum.

His advantage, said Plotkin, is that voters believe Romney is the candidate who is most likely to sway independent voters.

"I would think that by temperament and by political disposition, Romney is more likely to get independent voters," he said, adding that independents make up for about 20 per cent of the votes in a national election.

That's not to say that Romney is the perfect candidate, said Plotkin.

"There just is a lack of passion and intensity and excitement about the Romney candidacy but he has organization and he has money in the bank," he said.

Still, Romney's rivals continue to hit hustings across New Hampshire -- with stops scheduled in Nashua, Bedford, Salem and more -- in hopes of disrupting the frontrunner's lead on Tuesday.

Plotkin said more right-wing candidates such as Santorum or Gingrich could benefit from conveying a more moderate image.

"They would immediately have to, if they were the nominee, go to the centre to win a general election," he said.

Congressman Ron Paul came in third in Iowa, followed by Gingrich in fourth and Texas Gov. Rick Perry in fifth place. Last place went to lone female candidate Michele Bachmann who pulled out of the race after her poor showing.

The next anticipated primary is in South Carolina on Jan 21. Texas governor Rick Perry is skipping New Hampshire in hopes of doing well in the Southern state.

With files from The Associated Press