COLUMBIA, South Carolina: Presidential candidate Mitt Romney stood alone Monday at the top of the field of Republican candidates in this week's primary balloting in South Carolina, where three of his more-conservative challengers are splitting the powerful block of evangelical and social conservative voters.

Former Utah Gov. Jon Huntsman, by far the most moderate Republican candidate vying for the nomination to face President Barack Obama in November, dropped out of the race Monday and endorsed Romney.

That leaves just five men in the competition heading into South Carolina's critical Saturday vote, and a Romney victory likely would make his nomination inevitable.

Romney previously was first in Iowa caucus voting, then in the New Hampshire primary polling last week.

While a majority of Republicans don't trust Romney because of his moderate past, they have been unable to settle their support on the more-conservative alternatives: Newt Gingrich, the former speaker of the House of Representatives; Rick Santorum, a former Pennsylvania senator, or Texas Gov. Rick Perry. Small-government libertarian candidate Ron Paul, a Texas congressman, likewise has failed to rally sufficient support to make him a serious challenger to Romney in the first primary vote in America's deep South.

Republicans now seem to be slowly converging on Romney as the best of the Republican field to deny Obama a second term in the White House. Obama's incumbency is heavily weighed down by the struggling U.S. economy that has been slow to pull out of the Great Recession of 2007-2009. Unemployment remains high, and millions of Americans have lost their homes to mortgage foreclosures.

The candidates faced a packed week of campaign events and nationally televised debates Monday and Thursday. In the past three decades, no Republican has won the party's presidential nomination without carrying South Carolina.

Polls show Romney, the former Massachusetts governor who struggled to a fourth-place finish in South Carolina during his 2008 White House run, with a comfortable lead heading into Saturday's vote. The state has a large population of evangelicals and other conservative Christians, and concerns arose four years ago about his Mormon faith.

But Gingrich, Santorum and Perry all said Romney, after his victories in Iowa and New Hampshire, continued to benefit from the fractured Republican field and the failure of social conservatives to coalesce around a single alternative.

That has left Romney in control of the race, despite a lack of support from conservatives who are put off by Romney's shifting stances on social issues like abortion.

"I think the only way that a Massachusetts moderate can get through South Carolina is if the vote is split," said Gingrich on Sunday, portraying himself as the lone conservative with a "realistic chance" of beating Romney in South Carolina.

Huntsman, the former Utah governor and Obama's ambassador to China, will withdraw Monday, campaign manager Matt David told The Associated Press on Sunday. He will endorse Romney at an event in South Carolina, campaign officials said.

Huntsman placed third in last week's New Hampshire primary despite devoting most of his campaign resources to the state.

His resume suggested he could be a major contender for the Republican nomination: businessman, diplomat, governor, veteran of four presidential administrations, an expert on China and on foreign trade.

But Huntsman, who tried to avoid leveling attacks on opponents and Obama, was routinely at the bottom of national polls, barely registering at 1 percent or 2 percent.

He is dropping out as pressure also has been increasing on Perry to leave the race in order to allow South Carolina's influential social conservatives to unify behind Santorum or Gingrich.

Gingrich said he would "reassess" his candidacy if he lost in South Carolina and acknowledged that a Romney victory would mean "an enormous advantage going forward."

The field that remains after Saturday's vote will next compete in Florida on Jan. 31. The party does not officially name its candidate until the Republican National Convention in September.