Mitt Romney has won the New Hampshire primary, as he attempts to firm up support and extend his lead in the race to become the next Republican presidential candidate.

While Romney had all but locked up the New Hampshire presidential primary before the vote, the margin of victory is considered crucial as Romney looks ahead to the all-important primary in South Carolina later this month.

It's there that contenders like Ron Paul, Jon Huntsman and Newt Gingrich aim to make a stand against the Republican front-runner.

Results from the New Hampshire gave Romney, the former governor of Massachusetts, a solid win following his first-place finish last week in Iowa.

Other contenders, like Texas Rep. Ron Paul and Utah Gov. Jon Huntsman, were also doing battle with Gingrich, the former House speaker, and Rick Santorum, the ex-senator from Pennsylvania.

With 85 per cent of polls reporting, the results Tuesday night were:

  • Mitt Romney – 39.2 per cent
  • Ron Paul – 22.9 per cent
  • Jon Hunstman – 16.8 per cent
  • Newt Gingrich – 9.5 per cent
  • Rick Santorum – 9.4 per cent

Romney's main contenders have already begun to shift to the next stage of the race, which is on Jan. 21 in South Carolina.

David Frum, a former aide to president George W. Bush and current columnist for Newsweek, says Romney has proven in Iowa he can beat his most conservative opponents and New Hampshire will reveal the strength of his competition.

"How much of a serious opposition does he have?" Frum asked on CTV's National Affairs. "Romney will come out of this state with Ron Paul as his last opponent.

"Nothing could be better than that (for Romney.)"

Romney lost the South Carolina primary to Senator John McCain in 2008 but polling puts him in the lead in the staunchly conservative state over both Gingrich and Santorum.

A win in that state could make his nomination close to inevitable. The Republican establishment is hoping to avoid a long drawn-out race in order to go after U.S. President Barack Obama as soon as possible.

South Carolina has voted for every Republican presidential nominee since 1980.

However, national polls have put Romney with only about 25 to 30 per cent among Republicans.

After his narrow win in Iowa, Romney has managed to hang on to his lead, but his rivals have ramped up attacks against him. On Monday, a pro-Gingrich group -- not affiliated with the candidate -- unveiled a scathing ad campaign which criticizes Romney's history as head of Bain capital, a venture capital firm.

Romney, who won Iowa with only eight votes over Santorum, had to spend the latter part of his New Hampshire campaign on the defence after making a comment about firing people.

"I like being able to fire people who provide services to me," he was quoted as saying when asked about a health care program that would allow people to purchase their own insurance.

Romney defended the remark Tuesday, saying he was referring to getting rid of an unsatisfying health insurance provider, but it appears the damage is already done.

When Romney reached out to hold a supporter's baby at an event Tuesday, someone yelled: "Are you going to fire the baby?"

With files from The Associated Press