According to Bloomberg:
Huntsman, a former Utah governor who served as ambassador to China for President Barack Obama, struggled financially throughout the primary contest and was running last in an average of three statewide polls. He didn’t compete in the Jan. 3 Iowa caucuses and finished third on Jan. 10 in New Hampshire, the contest on which he had staked his campaign.
Matt David, a spokesman for Huntsman, told The New York Times and the Associated Press that Huntsman would endorse former Massachusetts governor and fellow Mormon Romney.
Word of the Huntsman withdrawal came on the eve of a presidential debate in Myrtle Beach and the same day The State, South Carolina's largest newspaper, endorsed him for president.
The endorsement said there were "two sensible, experienced grown-ups in the race," referring to Romney and Huntsman. But it said Huntsman "is more principled, has a far more impressive resume and offers a significantly more important message."
However, Huntsman, 51, is still placing at the bottom of most national and South Carolina polls in the low single digits, according to USA Today.
The AP wrote that Huntsman "was almost invisible in a race often dominated by Romney."