(REUTERS) - On the eve of a widely expected victory in New Hampshire, Republican front-runner Mitt Romney struggled to repel attacks on his business record Monday as rivals tried to weaken him before a tighter vote in South Carolina.

The former venture capitalist did himself no favors in the homestretch to Tuesday's first-in-the-nation primary with remarks that sounded like he enjoyed firing people - and rivals were quick to seize on his choice of words.

When telling business leaders about how he wanted individuals to be able to choose their own health insurance, Romney said: If you don't like what they do, you can fire them. I like being able to fire people who provide services to me.

In reaction, former Utah Gov. Jon Huntsman quipped: that was another thing that differentiates Gov. Romney from me. Gov. Romney enjoys firing people. I enjoy creating jobs.

Romney, a former governor of Massachusetts, was ahead more than 20 points in New Hampshire, even though polls early Monday showed a slight erosion in his lead over his nearest rivals. Libertarian Ron Paul retained the No. 2 spot, while the moderate Huntsman and social conservative Rick Santorum made gains.

The small northern New England state is likely to give Romney his second victory after an eight-vote squeaker over Santorum in Iowa last week, making him the first Republican candidate who is not an incumbent president to win both states. It will also give legs to the second-place finisher, who could be any of the five other candidates.

But much of the focus in the Republican campaign is already on the Jan. 21 primary in South Carolina, a more conservative state where Romney's rivals are planning a barrage of critical ads to try to derail his march to the party's presidential nomination.

A fundraising group supporting former House Speaker Newt Gingrich has said it will release a 27-minute documentary that portrays Romney as a job cutter when he was a venture capitalist in the 1990s. A three-minute trailer for the video was posted on YouTube as Gingrich stepped up his criticism of Romney.

Romney has sought to cast himself as a job creator, someone who can charge up the U.S. economy at a time when the jobless rate, though improving slightly, sits at 8.5 percent. It is a rate that could put Democratic President Barack Obama on shaky ground in this election year.

The flap over the firing comment threatened to drown out Romney's repeated defense of his time running Bain Capital, a firm that bought companies and restructured them.

I'm happy to describe my experience in the private economy and the fact that if you take all the businesses that we invested in over our many years over 100 different businesses that collectively net-net added over 100,000 new jobs, he told a news conference.

'CORPORATE RAIDER'

In a remarkable turnaround in a party known for being friendly to business, Republicans seeking to slow Romney down are sounding more like populists as they bash Romney's work as a venture capitalist.

Gingrich and other Republican rivals have dubbed Romney a corporate raider who killed jobs.

The trailer to the anti-Romney documentary, When Mitt Romney Came to Town, portrays Romney as being part of a band of ruthless businessmen.

Mitt Romney was not a capitalist during his reign at Bain. He was a predatory corporate raider, says the trailer, which like the film was produced by Winning Our Future, a pro-Gingrich group.

Texas Gov. Rick Perry chimed in from South Carolina, where he hopes to revive a flagging campaign that almost ended after a dismal showing in the Iowa caucuses.

There is something inherently wrong when getting rich off failure and sticking it to someone else is the way you do business, said Perry. It is the ultimate insult for Mitt Romney to come to South Carolina and say he feels your pain.

Romney also has a 12-point lead over closest rival Gingrich in Florida three weeks before that state's Republican primary, but more than half of likely voters still might change their minds, according to a poll released on Monday.

SLOWING DOWN ROMNEY

While many analysts suggest that Romney is on the verge of nailing down his nomination, an onslaught of money and attack ads could slow down a front-runner who has failed to capture conservatives in a fractured party.

Even before New Hampshire residents cast a vote, South Carolina is shaping up as the main battleground.

In the latest example of how so-called Super PACs, political action committees with no donation limits, are shaping the campaign, Winning Our Future plans to spend $3.4 million on ads in South Carolina, a source familiar with the PAC plans told Reuters on condition of anonymity.

After a sharp ad attack on Gingrich by a pro-Romney group helped drive down Gingrich's support before the Iowa caucuses, it is certain that Romney will be the main target of Winning Our Future's ads.

Romney weathered well a bruising debate on Sunday in which his opponents criticized his ability to defeat Obama in November and represent Republicans' conservative core.

In two New Hampshire debates over the weekend, Gingrich hit Romney with allegations that Bain destroyed companies and fired workers, erroneously crediting a story to The New York Times. The story, Romney's Steel Skeleton in the Bain closet, actually was published by Reuters.

Although Romney could win big in New Hampshire, where he has a vacation home, the rest of the field has a chance for the coveted second place, including Huntsman, who has staked his bid on the Granite State after skipping Iowa.

I predict Huntsman will come in very strong and leave the state with a lot of momentum, Robert Reese, 73, a retired real estate developer, said after attending a Santorum rally.

(Additional reporting by John Whitesides in South Carolina and Ros Krasny and Sam Youngman in New Hampshire; writing by Mary Milliken; editing by Mohammad Zargham)