Jon Huntsman suspended his bid for the Republican presidential nomination on Monday, calling on his party to end "an onslaught of negative and personal attacks" in the campaign and then endorsing rival Mitt Romney.

The former Utah governor, who also served as U.S. ambassador to China under Democratic President Barack Obama, never gained traction in the Republican campaign. Some conservative Republicans questioned his having worked for Obama; others doubted his conservative credentials.

Huntsman lagged in public opinion polls and had finished third in last week's New Hampshire primary despite focusing much of his energy there after skipping the first nominating contest in Iowa.

His departure from the race and endorsement could provide a modest boost for fellow Mormon Romney, who is ahead in South Carolina after winning in Iowa and New Hampshire.

Huntsman's announcement came just hours before a debate for Republican presidential candidates here, and trims the field to five candidates: former Massachusetts governor Romney, former Pennsylvania senator Rick Santorum, Texas congressman Ron Paul, former House speaker Newt Gingrich and Texas Governor Rick Perry.

Huntsman and Romney are longtime rivals for the hearts of moderate Republican voters, and Huntsman barely mentioned Romney's name in endorsing him. Romney was not present at Huntsman's announcement.

Instead, Huntsman harshly criticized the negative tone that the Republican race has taken on since Gingrich and other candidates put Romney in their sights after the Iowa caucuses on Jan. 3.

"This race has degenerated into an onslaught of negative and personal attacks not worthy of the American people and not worthy of this critical time in our nation's history," Huntsman said.

Voters in Iowa, New Hampshire and now South Carolina have been inundated with negative radio and TV ads about the candidates, Romney and Gingrich in particular.

Many of the negative ads in the campaign have been the work of "super" political action committees that support candidates but operate independently from the campaigns and can receive unlimited donations.

Gingrich and Perry have cast Romney as a "job killer" and a "vulture capitalist" for Romney's time with Bain Capital, the private equity firm Romney co-founded. Bain was involved in several corporate takeovers, some of which involved cutting costs by laying off workers.

In criticizing Romney's business record, Gingrich and Perry became the rare Republicans to attack free enterprise capitalism, a fact that didn't sit well with many conservatives.

Although Huntsman decried such tactics on Monday, he too had joined in the attacks on Romney at various times during the campaign.

Last week in New Hampshire, Huntsman seized upon remarks Romney made in a healthcare discussion in which Romney said that when it came to healthcare, he enjoyed firing people who didn't provide good service.

Without referencing the context of Romney's comments, Huntsman said the fact that Romney "enjoys firing people" showed that he was "so detached from the problems people face today" and was "unelectable."

David Axelrod, a senior adviser to President Obama's re-election team, noted on Twitter on Monday morning that Huntsman had once called Romney a "well-lubricated weather vane" who changes his positions based on the political winds.

Even so, politics have a way of quickly turning enemies into friends of convenience, and that appeared to be the case on Monday.

Huntsman urged the Republican Party to unite behind the candidate "best equipped" to beat Obama, and said Romney is that candidate.

Although political analysts and commentators speculated that Huntsman might make another run for president in 2016, his campaign would not comment and he took no questions after his announcement.

"Today our campaign for the presidency ends," Huntsman said, "but our campaign to build a better and more trustworthy America continues."