The former Utah governor coupled his announcement with an appeal to the remaining contenders to stop attacking one another in television commercials. "At its core, the Republican Party is a party of ideas, but the current toxic form of our political discourse does not help our cause," he said.
He noted that he and Romney have had differences, and he did not respond when asked if he still believes — as he said while campaigning for last week's New Hampshire primary — that the former Massachusetts governor is out of touch and unelectable.
Huntsman said he was suspending his candidacy, but his endorsement made it clear that was a euphemism. He dropped out less than a week after finishing third in New Hampshire, where he had staked his candidacy. While he has campaigned in South Carolina for nearly a week, he lacked the money to pay for TV commercials or other essentials of a modern campaign.
Given Huntsman's decision to back Romney, his departure seemed unlikely to clarify the overriding question of the Republican campaign, whether conservative voters could or would unify behind Rick Santorum, Newt Gingrich or Rick Perry to create a strong conservative challenger to Romney.
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 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, 51, was almost invisible in a race often dominated by Romney, a fellow Mormon. One reason was timing. Romney and other declared or expected-to-declare candidates drew media attention and wooed voters, Huntsman 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 in June in the shadow of the Statue of Liberty. The former Utah governor had already acknowledged "very low" expectations for him in South Carolina's primary this week. Word of his withdrawal spread 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 to allow South Carolina's influential social conservatives to unify behind either former Pennsylvania Sen. Rick Santorum or former House Speaker Newt Gingrich.
Santorum worked over the weekend at consolidating conservatives, trying to parlay into support in South Carolina the decision Saturday by an influential group of national Christian conservatives to back him.
"I think it's important that we eventually consolidate this race," Santorum told reporters Monday at a news conference in Columbia. He stopped short of urging Perry, who has shown little traction in South Carolina, to quit the race.
"That's up to the candidates themselves to decide," Santorum said.
To stand out in a crowded field, Huntsman positioned himself as a tax-cutting, budget-balancing chief executive and former business executive who was above partisan politics. That would prove to be a hard sell to the conservatives dominating the early 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 who said 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.
Republicans seemed wary of Huntsman because of his work in the Obama administration. While he cast his appointment in August 2009 as U.S. ambassador to China as answering the call to serve his country, critics grumbled that he had in fact been working for the opposition.
Huntsman was conservative in matters of taxes and the scope of the federal government, but was out of step with most conservatives in his support of civil unions for gay couples.
In the end, Huntsman didn't seem to register, despite the continuing search for an alternative to Romney or a winner against Obama. He was routinely at the bottom of national polls, barely registering at 1 or 2 percent.
His campaign put all its emphasis on the New Hampshire primary, hoping that face-to-face politicking in the first-in-the-nation primary would pay off with a strong second-place finish or a surprise victory in Romney's backyard. While other GOP candidates spent December in Iowa, the Huntsman campaign ignored its leadoff caucuses, where social conservatives were all but certain to give him short shrift.
Central to Huntsman's New Hampshire strategy was its open Republican primary, which allowed independents to vote along with declared party members. He gambled that he could attract moderate voters, Republicans and independents alike, by presenting himself as a successful conservative leader who wasn't interested in engaging in a culture war.
He called his third-place showing a "ticket to ride" to South Carolina, but his distant finish behind Romney and runner-up Ron Paul was widely regarded as lackluster.