CHARLESTON, S.C. — Jon Huntsman staked his presidential campaign on New Hampshire and his bid to become a legitimate competitor on distinguishing himself from front- runner Mitt Romney. But less than a week after a disappointing third-place finish in the Granite State’s GOP primary, Huntsman decided to quit the race and back Romney.

Huntsman will endorse Romney, officials said Sunday, because he believes Romney is the best candidate to beat President Barack Obama in November. Campaign manager Matt David said Huntsman will announce his withdrawal today at an event in Myrtle Beach, S.C.

Huntsman’s resume had suggested he could be a major contender for the Republican presidential nomination: businessman, diplomat, governor, veteran of four presidential administrations, an expert on China and on foreign trade. But the former ambassador to China in the Obama administration found a poor reception for his brand of moderate civility that he had hoped would draw support from independents, as well as party moderates.

Huntsman was almost invisible in a race often dominated by Romney, a fellow Mormon. One reason was timing. For months, Romney and other declared or expected to-declare candidates drew media attention and wooed voters in early primary states.

Huntsman, however, was half a world away, serving as ambassador to China until he resigned in late April. Nearly two more months would pass before his kickoff speech on June 22 in the shadow of the Statue of Liberty. The former Utah governor had already acknowledged that expectations for him in South Carolina’s primary this week will be “ very low.” Word of the Huntsman withdrawal came on the same day that The State, South Carolina’s largest newspaper, endorsed him for president.

Although Huntsman was viewed as having little chance of finishing strong in South Carolina, his endorsement of Romney could give the former Massachusetts governor, who leads in state polls, even more of the look of inevitability.

The move comes as pressure has been increasing on Texas Gov. Rick Perry to leave the race in order to allow South Carolina’s influential social conservatives to unify behind either former Pennsylvania Sen. Rick Santorum or former House Speaker Newt Gingrich.

To stand out in a crowded field, Huntsman positioned himself as a tax-cutting, budget balancing chief executive and former business executive who could rise above partisan politics. That would prove to be a hard sell to the conservatives dominating the early voting contests, especially in an election cycle marked by bitter divisions between Republicans and Democrats and a boiling antipathy for Obama.

Huntsman also tried to offer a different tenor, promising a campaign marked by civility.

“I don’t think you need to run down somebody’s reputation in order to run for the office of president,” he said.

While Huntsman was often critical of his former boss — he joined those saying Obama had failed as a leader — and occasionally jabbed at Romney, he spent more of his time in debates pushing his own views for improving the economy rather than thumping the president or his opponents.

In light of his work in the Obama administration, Republicans seemed wary of Huntsman. While he cast his appointment in August 2009 as U.S. ambassador to China as answering the call to serve his country, his critics grumbled that he had in fact been working on behalf of the opposition.