Story highlights He didn't want to "stand in the way" of candidate who can beat Obama, official says

Jon Huntsman made a "basic calculation," says CNN's John King

The former Utah governor will endorse Mitt Romney, a source says

Earlier Sunday, South Carolina's largest newspaper endorsed Huntsman

Republican candidate Jon Huntsman will drop out of the presidential race on Monday and endorse front-runner Mitt Romney, a senior official with the former Utah governor's campaign told CNN.

Huntsman finished third in last week's New Hampshire primary, a state in which he had staked his entire campaign, behind Romney and Texas Rep. Ron Paul.

"Governor Huntsman did not want to stand in the way of the candidate best prepared to beat Barack Obama and turn our economy around. That's Mitt Romney," the official said.

The former governor and U.S. ambassador to China made headlines when he decided to take his campaign to compete in the South Carolina primary, where he struggled to build support.

An American Research Group poll released Friday showed Huntsman with just 1% of likely GOP primary voters in the state.

"Jon Huntsman made a basic calculation here, let's be honest. 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 goodwill in the party, a little bit of leverage," said CNN Chief National Correspondent John King.

Despite his low polling numbers, South Carolina's largest newspaper, The State, endorsed Huntsman earlier on Sunday.

The paper's editorial board stated that while Romney is "more appealing" than the rest of the GOP field, Huntsman is "more principled, has a far more impressive resume and offers a significantly more important message."

The board praised what it called the "essential values that drive his candidacy: honor and old-fashioned decency and pragmatism."

R.C. Hammond, the spokesman for candidate Newt Gingrich, said that with Huntsman dropping out, "we are one step closer to a bold Reagan conservative winning the GOP nomination."

Huntsman, known as one of the moderate candidates, launched his presidential campaign on June 21.

Meanwhile, conservative GOP presidential hopefuls attacked Romney on Sunday to try to head off what two influential Republicans said could be a decisive third straight victory for the former Massachusetts governor.

In appearances on morning talk shows, Gingrich, Rick Santorum and Rick Perry all contrasted their conservative credentials with Romney's more moderate record in hopes that South Carolina voters will agree with their contention that Romney can't defeat Obama in November.

Debates on Monday in Myrtle Beach and Thursday in Charleston will almost certainly give voters their last chance to see all the remaining Republican candidates on one stage, as some are likely to drop out after Saturday's vote. CNN will broadcast the Charleston debate.

The Palmetto State has chosen the eventual Republican nominee every time since 1980, and Romney's victories this month in the first two contests of the nominating process -- in Iowa and New Hampshire -- were unprecedented by a non-incumbent GOP contender.

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."

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On the same program, Scott said: "If Romney wins South Carolina, I think the game is over."

Perry, the Texas governor who lags behind after poor performances in earlier debates, kept up his criticism 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" who looted North Carolina companies, Perry used less combative language Sunday on CNN's "State of the Union" but reiterated his accusation that it "came in and basically shut down" a Georgetown steel mill, taking away "a lot of money in management fees."

President Obama's campaign will certainly raise the matter, if Romney gets the Republican nomination, Perry noted.

"The issue's not going away," he said. "Now's the time to talk about it, not in September and October."

Gingrich, the former House Speaker, expressed a similar view on NBC's "Meet the Press" and the CBS program "Face the Nation," comparing the primary process to the National Football League playoffs that eventually produce two hardened teams to compete in the Super Bowl.

"Our nominee had better be capable of standing up there, telling the truth, enduring the negative ads and winning the vote," Gingrich said, insisting he is the more accomplished debater and stronger candidate than Romney.

On NBC, Gingrich also said he would release his tax return on Thursday and reiterated his challenge for Romney to do the same, continuing a campaign to get Romney to disclose details of his personal wealth.

Romney has said he has complied with legal disclosure obligations, but added he might release his tax returns or further information in the future.

Gingrich said it would be better for Romney to release his tax return now instead of having the issue come to a head in the fall amid a one-on-one campaign against Obama.

"We're all going to do everything we can to defeat Barack Obama," Gingrich said, adding that Obama's re-election "would lead him to be so arrogant and so aggressive that it's hard to imagine what would come out of a second term."

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.

"I was told that individual members are going to go out and do things with, you know, either endorsements or contacting people here in the state and across the states to support us and to help our cause," said Santorum, a former Pennsylvania senator who was virtually tied for fourth in New Hampshire with Gingrich.

The Christian conservative endorsement was intended to unite evangelical voters behind one candidate to avoid a split that would hand Romney a victory despite South Carolina's conservative pedigree.

"It would be helpful if everybody dropped out and I would win," Santorum said. "But, you know, the idea is, we're going to go through this process, people have the right to go out and make the case to the voters and then we'll see what happens."

Scott, who won election in 2010 with strong support from the tea party movement, told NBC that the impact of the evangelical vote -- a typically large bloc in South Carolina -- will be "huge" on Saturday.

"It's hard to find a single candidate that rallies all of the Christian voters in South Carolina and therefore that splintered approach will probably have a major impact in the state's primary," Scott said.

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. Another 25% said they would support Gingrich, putting Romney's lead within the poll's sampling error.

The survey indicates Rep. Ron Paul of Texas has 20% of the vote, Perry has 9%, Santorum has 7%, with 7% undecided.