Former US envoy to China Jon Huntsman's presidential hopes faced a do-or-die test Tuesday in New Hampshire, where he may have hit his stride too late against frontrunner Mitt Romney.

Outside a Manchester elementary school that served as a polling place, Huntsman told reporters he and wife Mary Kaye would "live with the results" of the small northeastern state's critical nominating primary.

"You wake up and you say, 'I've done everything humanly possible,' Mary Kaye has done everything humanly possible. We've given our hearts to New Hampshire. That is all you can do," he said.

The former Utah governor, scion of one of America's richest families, saw his support rise steadily in the final days before the primary and hoped for a surprisingly strong finish to give him a much-needed boost.

"Tonight I think we're going to prove the point that we've got the staying power and we've got the ability to bring this thing home," he told NBC television as voters braved New Hampshire's wintry chill to cast their ballots.

Huntsman, who skipped Iowa's caucus last week and has bet his presidential fortunes on a strong showing here, said he hoped to "beat the marketplace expectations" to prove he could run strong in the November elections against Democratic President Barack Obama.

A primary-day public opinion poll by Suffolk University in nearby Boston found the two-time former ambassador in third place with 16 percent of likely voters saying they would back him.

The survey, which had an error margin of 4.4 percentage points, still put him far behind former Massachusetts governor and millionaire venture capitalist Romney, with 37 percent, but hot on the heels of Representative Ron Paul, who sat at 18 percent.

Huntsman had 26 percent of independents, who under New Hampshire's rules can vote in the Republican primary, but fell short among registered Republicans, with just nine percent -- a liability heading into the fight for conservative South Carolina, which votes January 21.

Huntsman played up his cross-party appeal, telling NBC that "in order to beat Barack Obama, the bottom line... is you've actually got to convince some people who voted for Barack Obama last time to vote for you.

"That's just the mathematical reality," said Huntsman, who has held more than 170 rallies, town hall meetings and other events in New Hampshire over the past few months, with Mary Kaye often his strongest advocate.

But his rise in the polls came after he struck back hard in a Sunday debate at Romney's charge that his service as Obama's first envoy to Beijing should disqualify him from being the party's standard-bearer.

"I will always put my country first," said Huntsman, who made "Country First" his new slogan and vowed first to restore both the US economy and the US public's failed trust in its political elites.

Huntsman also slammed his fellow Mormon over jobs lost while Romney headed the Bain Capital investment firm and pounced on his rival's comment -- made while describing hopes that consumers could get the upper hand on health insurance firms -- that "I like being able to fire people."

Romney "enjoys firing people, I enjoy creating jobs," Huntsman charged, adding the comment showed his rival was "slightly out of touch."

On paper, Huntsman should have shone, but the conservative governor of a well-run state and two-time ambassador with business experience may have misjudged a Republican electorate eager more for red-meat attacks on Obama than his own steady-as-she-goes approach.

"I'm not gonna light my hair on fire," he said in October. "I don't think you have to be crazy to be in the Republican Party."

And in a much-noticed Tweet in August, Huntsman broke with the Republican base, saying: "I believe in evolution and trust scientists on global warming. Call me crazy."

Huntsman, who has not ruled out a 2016 run, had a tender response Tuesday when asked what would be a good finish here. One that "puts a smile on Mary Kaye's face," he replied.