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(CNN) -- Former Utah Gov. Jon Huntsman withdrew from the Republican presidential race Monday and endorsed front-runner Mitt Romney, while a supporter of Texas Gov. Rick Perry called for him to also drop out for the sake of conservative unity.
And Romney was again the biggest target on stage at a debate in South Carolina five days before that state's primary on Saturday. But his rivals traded a few shots at each other as they tried to position themselves as the best conservative hopeful to derail the front-runner.
South Carolina state Sen. Larry Grooms, a favorite of the conservative tea party movement, said in a statement that "It is apparent that Gov. Perry cannot win and has no viable strategy in moving forward."
"Remaining in the race at this point only serves to steer votes away from viable candidates," Grooms said, adding: "Now is the time for us to re-evaluate our choices and coalesce around a single candidate."
Earlier, Huntsman announced his withdrawal in South Carolina, site of the critical first Southern primary on Saturday. His decision could provide an extra cushion of votes for Romney, who is facing stiff conservative competition from conservatives Rick Santorum and Newt Gingrich.
"Our campaign for the presidency ends, but our campaign for a (better) American continues," Huntsman declared. "I believe it is now time for our party to unite around the candidate best equipped to defeat Barack Obama. Despite our differences and the space between us on some of the issues, I believe that candidate is Gov. Mitt Romney."
In the debate at Myrtle Beach, former Pennsylvania Sen. Rick Santorum called for Romney to denounce an ad from a political action committee backing Romney that Santorum said distorted his vote on allowing convicted felons voting rights after they had served their time. Then he put Romney on the defensive by challenging his record on the issue while he was governor of Massachusetts.
Former House Speaker Newt Gingrich continued to question Romney's record while he headed a venture capital firm, something that has made fellow Republicans uneasy. Gingrich said that since Romney had made his business experience a key part of his pitch to voters, it was fair to ask for details.
And Perry challenged the front-runner to release his tax records. Perry said he might do so around April.
Gingrich had tried to make the case during the day that a vote for Santorum was essentially a vote for Romney because it would split conservatives and hand the primary to the former Massachusetts governor.
But after the debate, Santorum fired back at Gingrich, pointing out that he had beaten the former speaker in the first two contests in Iowa and New Hampshire.
The field will take the stage one more time before the Saturday's vote in a Thursday debate on CNN.
Gingrich used his speech at the South Carolina tea party convention earlier in the day to try to cast himself as the only conservative able to prevent the more moderate Romney from winning the nomination.
"The key question we have to decide on Saturday is whether we're going to nominate, from South Carolina, a Reagan conservative tough enough and strong enough to stand up to Obama and then to stand up to Washington. Or whether we're going to have conservatives split three ways and nominate somebody who had no history in Boston of changing the culture of Massachusetts at all," Gingrich told a tea party convention in South Carolina. "To do that, we have to unify the tea parties and we have to unify the conservatives in a very straightforward way."
He earlier challenged the argument that nominating Romney, who lost to unsuccessful GOP presidential contender John McCain in the 2008 primaries, gives Republicans the best chance of defeating President Barack Obama in November.
"You are arguing that the principle of nominating the guy who lost to the guy who lost is a good principle because we ought to sustain our tradition," Gingrich told reporters, adding: "We are a conservative party, but we don't have to be a stupid party."
Santorum also made the case that only a strong conservative -- in this case him -- can make the compelling case to voters in a race against Obama.
"We need a conviction conservative -- someone who is not just conservative on one issue and not just for one time, but someone who has been a conviction conservative on all of the issues, all of the time," Santorum said at the tea party convention in obvious reference to Romney's history of shifting positions on some major issues such as abortion.
Also Monday, Santorum called Huntsman's withdrawal and endorsement of Romney unsurprising.
"Gov. Huntsman ran as a moderate trying to compete with Gov. Romney for the establishment moderate vote," Santorum said at a campaign event in Columbia, South Carolina. "Gov. Romney had a leg up on him as being a solid moderate that the establishment could get behind, and Gov. Huntsman was unable able to crack through that."
In announcing his withdrawal, Huntsman urged the remaining candidates to stop attacking each other, warning that "the current toxic form of our political discourse does not help our cause."
"This race has degenerated into an onslaught of negative and personal attacks not worthy of the American people and not worthy of this critical time," he said.
Romney's campaign already is bolstered by a huge war chest and a wave of momentum after wins in Iowa and New Hampshire. It's hoping for a possible knockout blow with a first place-finish in the Palmetto State. South Carolina has picked the winner of every GOP presidential nomination fight since 1980.
Huntsman's endorsement of Romney represented a notable shift, after Huntsman recently declared Romney "completely unelectable" during an interview on CNBC.
"Jon Huntsman made a basic calculation here, let's be honest," said CNN Chief National Correspondent John King. "If you look at the polling, he's doing miserable in South Carolina. He's not going to win the state next Saturday and so if he wants to run in 2016 ... this is a chance ... to try to get a little bit of good will in the party, a little bit of leverage."
An American Research Group poll released Friday showed Huntsman with 1% of likely GOP primary voters in South Carolina.
Huntsman -- a former ambassador to China -- surprised a number of political observers when he initially decided to compete in South Carolina following his distant third-place finish in the New Hampshire primary.
While Huntsman characterized his New Hampshire finish as a victory, most pundits saw it as a disappointment. The former governor had devoted virtually all of his time and resources to the Granite State.
Meanwhile, Texas Rep. Ron Paul's campaign took in $1.3 million in a two-day, online fund-raising drive over the weekend that followed the libertarian champion's strong second-place finish in New Hampshire.
In the fund-raising effort now extended into the week, contributors are allowed to donate up to the legal maximum of $2,500 to Paul's website, where a real-time tracker displays the amount of money as it comes in, as well as donor names.
Paul is running third in South Carolina, behind Romney and Gingrich, according to the latest poll.
On Sunday, Gingrich and the other remaining conservative hopefuls stepped up their attacks on Romney to try to head off a third straight win for the former Massachusetts governor.
In appearances on morning talk shows, Gingrich, Santorum and Perry all contrasted their conservative credentials with what they characterized as Romney's more moderate gubernatorial record.
Debates on Monday in Myrtle Beach and Thursday in Charleston will likely give voters their last chance to see the five remaining Republican candidates on one stage, as some are considered likely to drop out after Saturday's vote. CNN will broadcast the Charleston debate.
Two South Carolina Republican politicians, Sen. Lindsey Graham and Rep. Tim Scott, said Sunday that another Romney victory this week would likely sew up the nomination for him.
"If for some reason he's not derailed here and Mitt Romney wins South Carolina, no one's ever won all three, I think it should be over," Graham told NBC's "Meet the Press" program. "That would be quite a testament to his ability as a candidate and a campaigner."
On the same program, Scott said: "If Romney wins South Carolina, I think the game is over."
Perry, who has lagged in the polls since a series of poor debate performances last fall, kept up his criticism Sunday of Romney's experience as a venture capitalist -- an attack line shared by Gingrich but criticized by Santorum and other conservatives.
After previously describing Romney's former company, Bain Capital, as corporate "vultures," Perry used less harsh language during an appearance on CNN's "State of the Union." He reiterated his accusation, however, that Bain "came in and basically shut down" a South Carolina steel company, taking away "a lot of money in management fees."
Gingrich, meanwhile, told the NBC program he would release his tax returns on Thursday, and reiterated his challenge for Romney to do the same -- continuing a campaign to get Romney to disclose details of his substantial personal wealth.
Santorum, meanwhile, told "Fox News Sunday" that an endorsement on Saturday from Christian conservative leaders should help his campaign, as he seeks to regain the luster of a razor-thin second-place finish behind Romney in Iowa.
Santorum, who finished fifth in New Hampshire, has been fighting with Gingrich and Perry for support from South Carolina's powerful evangelical voters. Saturday's endorsement was intended to unite those voters behind one candidate and avoid a split that would hand Romney a victory despite South Carolina's conservative pedigree.
The American Research Group poll released last week finds Romney and Gingrich in a statistical dead heat in the state.
According to the poll, 29% of likely GOP primary voters say they will support Romney. An additional 25% said they would support Gingrich, putting Romney's lead within the poll's sampling error.
The survey indicates Paul has 20% of the vote, Perry has 9%, Santorum has 7%, and 7% are undecided.
CNN's Peter Hamby, Alan Silverleib, Rachel Streitfeld, Jessica Yellin, Jim Acosta and Tom Cohen contributed to this report.