MYRTLE BEACH, S.C. — The Republican presidential contenders on Monday campaigned their way into the first of two debates before a pivotal weekend primary in South Carolina, with Mitt Romney savoring an endorsement from the latest campaign dropout and his pursuers struggling to emerge as the race’s principal conservative.

Hours before the debate, former Utah Gov. Jon Huntsman withdrew from the race and announced his support for Romney despite their differences. He appealed to all remaining contenders to stop attacking one another.

There appeared little likelihood of that happening, either in the TV commercials, mail and other advertising blanketing the state ahead of Saturday’s vote or, possibly, on the debate stage itself.

Romney wasn’t present for Huntsman’s endorsement, and Newt Gingrich, Rick Santorum, Rick Perry and Ron Paul all but ignored it as they sought to slow the front-runner’s momentum in the race to pick a Republican rival to President Barack Obama this fall.

Romney has victories in the only two contests of the campaign thus far, the Iowa caucuses and New Hampshire primary earlier this month. Gingrich has conceded that the former Massachusetts governor will likely be the party’s nominee if he is similarly victorious in South Carolina, an assertion that none of the others in the race has so far contested.

That raised the significance of the night’s debate, as well as another one scheduled for Thursday in Charleston.

Romney is the leader in the public opinion polls in South Carolina, though his rivals hope the state’s 9.9 percent unemployment rate and the presence of large numbers of socially conservative evangelical voters will allow one of them to slip by him.

Huntsman was the second campaign dropout to endorse Romney, after former Minnesota Gov. Tim Pawlenty. Minnesota Rep. Michele Bachmann, who quit after a last-place finish in Iowa, has not yet said which of the remaining contenders she supports. Herman Cain, who left the race in December after facing allegations of sexual impropriety, has promised an endorsement soon.

Huntsman’s parting announcement included a reference to the differences he and Romney had. But he left the podium without responding to questions about his remark last week, in the run-up to the New Hampshire primary, that Romney was unelectable and out of touch.

It was unclear why Romney did not attend the announcement. He was in town for a later campaign appearance and then the debate.