(Bloomberg News) Republican presidential candidate Newt Gingrich is trying to prove he's got something his rivals don't: staying power.

After a surge in the polls that revived a candidacy long dismissed as nothing more than a promotional book tour, the former House speaker is positioning himself as the party's strongest alternative to front-runner Mitt Romney.

"When you have a 90 percent American Conservative Union rating for your entire lifetime," he told reporters yesterday in Bluffton, South Carolina, where he was opening a new campaign office, "there's a clear contrast."

Gingrich's jab at the former Massachusetts governor, coming after months of urging his rivals to focus their criticism solely on President Barack Obama, signaled the start of a new phase of his campaign in a primary that has been notable for its lack of friendly fire.

On a three-day campaign swing through South Carolina, Gingrich packed town-hall meetings, wooed fundraisers, courted state party officials and served notice to his rivals that he is prepared to fight to hold his newfound front-runner status.

An American Research Group survey released yesterday showed 33 percent of likely South Carolina Republican primary voters backing Gingrich, up from 8 percent in October, compared with 22 percent supporting Romney.

Romney made his first direct attack on Gingrich last night, signaling the growing threat posed by his candidacy. "I think to get President Obama out of office, you're going to have to bring something to the race that's different than what he brings," Romney said. "He's a lifelong politician."

Re-energized National Strategy

Gingrich's South Carolina visit illustrated the campaign's re-energized national strategy. Book signings have been replaced with town-hall meetings. Field teams and state-specific websites are being launched in New Hampshire, Iowa, and South Carolina. And the campaign says it has raised more than $4 million since the end of September, enough to infuse a new sense of confidence about its ability to compete in a drawn-out primary.

"If we do well in South Carolina and win in Florida, down goes Willard," said R.C. Hammond, Gingrich's campaign spokesman, referring to Romney by his formal first name.

Gingrich is working to avoid the fate of some of his rivals, who surged in the polls only to plummet after weak debate performances or other self-inflicted errors. Herman Cain, the former chief executive officer of Godfather's Pizza who is facing sexual misconduct allegations, has been the latest victim of that boom-bust cycle. A Nov. 14 CNN poll found that Cain's support among Republicans fell to 14 percent from 25 percent in October.

Cain Reassessing Candidacy

A new accusation this week that Cain was involved in a sporadic, 13-year affair prompted the Atlanta businessman to announce that he is reassessing his candidacy, a move that could benefit a rising Gingrich.

Evidence of the former speaker's revival since June, when more than a dozen top advisers quit citing disputes over campaign strategy, is growing.

Some aides who remained, such as Hammond, had resorted to sleeping in supporters' homes, bunking together above garages or in spare bedrooms, when fundraising slowed to a trickle in the summer. As of Sept. 30, Gingrich was $1.2 million in debt, according to campaign disclosure reports.

South Carolina Offices

Today, the Gingrich campaign has 10 paid staff and has opened five offices in South Carolina. In New Hampshire, six aides are working out of a new office in the center of downtown Manchester. And in Iowa, Craig Schoenfeld and Katie Koberg, two of six staffers in the state who resigned on June 9, rejoined the campaign mid-November as senior advisers. Hammond has now stayed in enough hotels to earn frequent guest status with Marriott Hotels.

"We first started going across the state in early September," said James Epley, the campaign chairman for Beaufort County, South Carolina. "We've seen it grow from there."

Still, with less than five weeks before the first round of voting in Iowa, some Republicans question whether Gingrich has enough time to build an effective operation.

"The short time frame will make whatever it is less effective," said Jim Dyke, a South Carolina-based Republican strategist. "Highly motivated volunteers can trump a lot of things -- just not time."

Immigration Issue

As Gingrich works to catch up organizationally, he also must defend the sort of debate remarks that have gotten his competitors into trouble. When Texas Governor Rick Perry was deemed too soft on illegal immigration after a Sept. 22 debate that highlighted his support for in-state tuition of American- born children of undocumented workers, he lost his front-runner status and has never recovered.

Gingrich spent much of the week defending his plan to grant legal status to some immigrants who entered the country illegally long ago, after Romney and Minnesota Representative Michele Bachmann labeled it "amnesty" during a Nov. 23 candidate forum in Washington.

"It is an absolute falsehood to say that I favor amnesty for 11 million people. Period," Gingrich told voters gathered on a town green in Bluffton, South Carolina.

"Anybody who says it from now on has been served notice that they are saying something which is not true, which in itself should disqualify them as a candidate," he said.

The former speaker also emphasized to voters that he didn't work as a lobbyist after leaving office. Gingrich made millions through a network of advocacy organizations, think tanks and consulting firms he founded after resigning the speakership and his House seat in 1999.

Freddie Mac Ties

His clients included Freddie Mac, a mortgage company taken into government conservatorship in 2008 after its stake in failed subprime loans pushed it to the brink of collapse.

"I can't tell you which distortion my opponents are going to raise," he said. "I can tell you this: I will be prepared to answer every one of them."

Republican Representative Tim Scott, who hosted a town-hall meeting with Gingrich in Charleston, said the presidential hopeful's immigration position will likely be a liability with the state's conservative Republican voters.

To ease that opposition, Scott is urging voters to take a broader look at Gingrich's experience. "You don't have to find the perfect candidate," he said, in an interview on CNN's "State of The Union." "What we need is someone who can beat President Obama."

Newspaper Endorsement

New Hampshire's Manchester Union Leader's Nov. 27 endorsement of Gingrich took a similar position. "We would rather back someone with whom we may sometimes disagree than one who tells us what he thinks we want to hear," the newspaper wrote.

In appearances across South Carolina, Gingrich said that his shortcomings don't mean that he isn't the best candidate to take on Obama.

"I don't claim to be the perfect candidate," he told WSC- FM radio on Nov. 28. "I just claim to be a lot more conservative than Mitt Romney, and a lot more electable than anybody else,"

Gingrich promised retirees in Bluffton, a few miles from Hilton Head, that his general election campaign would deliver a "crushing defeat" to Obama.

Still, the former speaker's background bothered some voters, who said his past advocacy work, two divorces and admissions of infidelity would make it difficult for him to win in 2012.

"My concern is that Gingrich has had a high negative image in many, many areas in the past," said Jim McGrath, of Sun City, South Carolina. "How is he going to overcome that with the independent voter?"