(Reuters) - Jon Huntsman will drop out of the Republican presidential race on Monday and endorse Mitt Romney, a senior Huntsman campaign official said Sunday night.

Huntsman's campaign never took off. Conservative Republicans mistrusted him for working for President Barack Obama as U.S. ambassador to China.

Huntsman is proud of the race he ran, the campaign official told Reuters on Sunday. A source close to Huntsman said he would make the announcement in Myrtle Beach, South Carolina, the state where Republican hopefuls are campaigning for Saturday's primary as they select a candidate to challenge Obama in the November election.

Ultimately, he didn't want to stand in the way of the person with the best chance to beat President Obama, the official said.

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

A relatively moderate Republican, Huntsman lagged in opinion polls and finished third in last week's New Hampshire primary despite a huge campaigning effort there.

A poll last week showed Huntsman at only 4 percent in South Carolina, behind even satirical talk-show host Stephen Colbert.

Huntsman served two terms as governor of Utah. He was also a deputy U.S. trade representative in former President George W. Bush's administration.

In the 2008 race for the Republican presidential nomination, Huntsman supported Arizona Sen. John McCain, rather than Romney, a fellow Mormon.

With South Carolina's primary approaching, time is running short for other candidates to slow Romney's march to the Republican nomination.

The winner of South Carolina's primary has gone on to capture the Republican nomination in every election since 1980.

(Additional reporting by JoAnne Allen in Washington and Michelle Nichols in New York; Editing By Peter Cooney)