With Mitt Romney poised to stride toward the Republican presidential nomination by capturing New Hampshire on Tuesday, all eyes were on whether he could win big enough to convince his party he is the best candidate to defeat Democrat Barack Obama.

Despite rivals' fierce eleventh-hour attacks that he is a heartless corporate raider who enjoys cutting jobs, the former governor of neighbouring Massachusetts carried a sizeable poll lead into primary day.

Romney's cushion was sufficient enough to force leading rivals Ron Paul, who appeals to New Hampshire's anti-government strain, and Jon Huntsman, who has campaigned almost exclusively in the state, into a battle to be a strong runner-up.

A Reuters/Ipsos poll on Tuesday showed Romney was way ahead of rival Republicans nationally, although he still trailed Obama by five points in the White House race. Thirty percent of Republicans in the poll would vote for Romney, compared to 18 percent a month ago.

But Romney is gaining on Obama. In December he trailed the president by an eight percentage point margin in the same poll.

Romney would be the first Republican who is not an incumbent president to win the first two early voting states, after his slim eight-vote victory over former Pennsylvania Senator Rick Santorum a week ago in Iowa.

A resounding win would provide momentum going into South Carolina on January 21 and Florida on January 31. Romney leads in polls of both states and victories there would all but sew up his nomination to face Obama as he seeks re-election on November 6.

It was unclear how much damage had been done by a mess of his own making in which Romney declared on Monday: I like being able to fire people who provide services to me, in discussing the need for greater competition between health insurance companies.

In a sharp departure for a party known as friendly to business, Republicans seeking to slow Romney seized on the comment to bash his time at Bain Capital, a firm that bought companies and restructured them.

Gingrich has launched the toughest onslaught.

Mitt Romney was not a capitalist during his reign at Bain. He was a predatory corporate raider, When Mitt Romney Came to Town, a 27-minute video produced by a pro-Gingrich group, said.

Results should start flowing in shortly after New Hampshire voting stations close at 7 p.m. EST (midnight GMT).

(Additional reporting by Ros Krasny in Manchester and Jeff Mason in Washington; Writing by Patricia Zengerle)