* Reduced field of five will meet in South Carolina

* Romney likely to face attacks (Adds start of debate)

By John Whitesides

MYRTLE BEACH, S.C., Jan 16 (Reuters) - A newly trimmed field of five Republican presidential hopefuls opened a South Carolina debate on Monday that gives front-runner Mitt Romney's rivals one of their final chances to derail his growing momentum.

The debate came hours after former Utah Governor Jon Huntsman dropped out of the 2012 Republican race and endorsed Romney, bolstering the former Massachusetts governor's already formidable lead for his party's nomination. Romney won the first two state nominating contests in Iowa and New Hampshire this month.

It also provided Rick Santorum and Newt Gingrich a prominent stage for their battle to become the top conservative alternative to the more moderate Romney.

Santorum, a former U.S. senator from Pennsylvania, gained valuable backing for that position over the weekend when a group of 150 religious and social conservative leaders agreed to coalesce behind his candidacy in an effort to stop Romney.

The debate is the first of two to be held this week in South Carolina, where a Romney win in Saturday's primary could put him on an almost certain path to clinching the right to challenge President Barack Obama in November's election.

Polls show Romney with a solid lead in South Carolina over Gingrich, the former U.S. House of Representatives speaker, heading into the debate. Another debate will be held in Charleston on Thursday, less than 48 hours before South Carolina Republicans start to vote.

"At this point, Romney just has to remind people that he is the one who can take on Obama and win in November," Republican consultant Rich Galen said. "He can't let the other guys get under his skin."

In addition to Romney, Gingrich and Santorum, Texas Governor Rick Perry and U.S. Representative Ron Paul are also participating in Monday's two-hour debate in Myrtle Beach, South Carolina.

SHARP ATTACKS ON ROMNEY

The last Republican debates were back-to-back meetings within 12 hours of each other before the New Hampshire primary, which Romney won easily after narrowly winning in Iowa.

Those debates featured several sharp attacks on Romney for his work at a private equity firm that critics say plundered companies and slashed jobs, but his Republican rivals have eased off those criticisms in recent days.

South Carolina's 9.9 percent unemployment rate is higher than the national average of 8.5 percent, making jobs and unemployment one of the most prominent topics likely to be tackled in the debate.

Santorum, Gingrich and Perry have pursued South Carolina's large bloc of evangelical and social conservative voters, who have been split here, as they were in Iowa.

Santorum, who came in second in Iowa, and Gingrich have argued they are the most electable conservatives, but neither have shown signs in polls yet that they are breaking through in South Carolina, which could be the last chance to stop Romney.

The next battleground after South Carolina will be Florida on Jan. 31, a huge and diverse state where Romney's financial and organizational advantages would make him hard to stop.

"Romney seems to be pulling away if the polls are to be believed," Galen said. "But he can't put it on cruise control yet."

(Editing by Philip Barbara)