(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.