COLUMBIA, S.C. — Republican Jon Huntsman's endorsement of Mitt Romney further cements Romney as the favorite in Saturday's pivotal South Carolina primary, adding another name to the list of party figures calling him best able to beat Democratic President Barack Obama.
However, it leaves Romney's more conservative rivals still fighting with one another to emerge as his strongest challenger. And it prompted at least one backer of Texas Gov. Rick Perry to urge him to quit the race.
Besides offering Romney his support, Huntsman also appealed for civility in a race that has become defined by attack advertisements paid for by anonymous groups.
"At its core, the Republican Party is a party of ideas, but the current toxic form of our political discourse does not help our cause," he said, as televisions and telephones across the state hummed with attack messages.
Huntsman's endorsement offers little in the way of a campaign organization or bankable support to Romney.
But it does add to the sense of inevitability the former Massachusetts governor has worked to portray in a state that has voted for the eventual GOP since 1980 and where he is campaigning with backing from the state's new Gov. Nikki Haley and Arizona Sen. John McCain, winner of the 2008 primary here.
Romney stands a good chance of winning the votes Huntsman would have received. Both tout business backgrounds and are social moderates in a state where social conservatives are an influential bloc. Polls show Romney was most often the second choice of Huntsman backers.
"Certainly, it will help Gov. Romney here, it's just not clear how much," said former state Attorney General Henry McMaster. An early Huntsman supporter, McMaster had not committed to another candidate after Huntsman's announcement.
But Huntsman's departure leaves unanswered the overriding question of the Republican campaign: whether voters looking for a candidate more conservative than Romney will unite behind Rick Santorum, Newt Gingrich or Perry and send a strong challenger to compete against Romney in the Florida primary or beyond.
Santorum, a former Pennsylvania senator, dismissed the endorsement during a campaign stop at a Columbia restaurant. "Moderates are backing moderates," he said. "No surprise there."
Gingrich, the former House speaker locked in a bitter battle with Romney, said at an appearance in Myrtle Beach: "Why would you want to nominate the guy who lost to the guy who lost to Obama?"
Gingrich also quickly noted late Sunday, as news of Huntsman's decision broke, that a Greenville businesswoman who had supported the former Utah governor was coming over to Gingrich.
Although Huntsman's departure hardly assures Romney of victory in Saturday's primary, it underscored the tension within the party's conservative wing with just five days left for one of them to emerge.
Santorum was the consensus, if not unanimous, choice of a group of national Christian political activists who met over the weekend in a last-ditch effort to rally conservatives.
"I think it's important that we eventually consolidate this race," Santorum said, stopping short of saying pressure was building on Perry to drop out. "That's up to the candidates themselves to decide."
Polls in South Carolina showed Romney in the lead, followed by Gingrich with Santorum and Texas Rep. Ron Paul fighting for third. Perry trailed in single digits.
A few of Perry's backers have begun calling on him to leave the race.
"There are a lot of conservatives who were happy to see him get in and now who would be happy to see him get out," state Rep. Larry Grooms, an early Perry supporter, told The Associated Press. "When conservatives have split in the past, we end up nominating a moderate, and that's not good for our party."
Conservative author Eric Metaxas indirectly suggested Perry should quit the race while speaking at a packed prayer breakfast in Myrtle Beach on Sunday. The night before he had tweeted: "Dear Gov. Perry: Do the right thing for your country; endorse Rick Santorum before the SC Primary next Saturday ... you'll wish you had."
Some South Carolina Republicans still rue McCain's narrow 2008 win over the more socially conservative Mike Huckabee, allowed largely by former Tennessee Sen. Fred Thompson's decision to stay in the race.
Katon Dawson, Perry's South Carolina chairman, resisted the idea that there was a drumbeat for Perry to quit. "I find it sort of offensive that some Republicans would tell us they want to hand pick the candidate before the election," Dawson said.
Ralph Reed, founder of the national Faith and Freedom Coalition, also said he knew of no effort by the conservative leaders who met Saturday in Texas to urge Perry to quit.
"We have never asked any candidate to drop out of the race publicly or privately. And we will not do so," Reed said.
Some evangelical pastors and conservative leaders in Iowa had asked Minnesota Rep. Michele Bachmann to consider ending her campaign to allow a conservative to emerge. She did not, and Romney edged Santorum in Iowa by a razor-thin eight votes.
With Perry showing no signs of departing, Santorum faces the added burden of a barrage of attack ads by an independent super PAC that supports Romney's campaign.
The group, Restore Our Future, was running almost $900,000 worth of advertising this week, more than any of the candidates' campaigns and all of it against Santorum.
Santorum complained Monday about the ads, which accuse him of having supported earmark spending in Congress and supporting voting rights for felons.
Santorum has defended his support for earmark spending. He said Monday he supports allowing felons who have completed their sentences to apply to have their voting rights restored, a policy that is law in 48 states.
Restore Our Future spent roughly $3 million on similar anti-Gingrich ads in the final month of the Iowa campaign and was seen as contributing to his fourth-place finish in the caucuses.
A similar group that supports Gingrich was running roughly $850,000 in ads against Romney.
Hunt reported from Myrtle Beach, S.C. Associated Press writer Shannon McCaffrey in Myrtle Beach, S.C., contributed to this report.