NEWBERRY, S.C. — For three decades, the Republican who won South Carolina’s presidential primary has also won the GOP nomination.

That record helps explain why Newt Gingrich, a self-described lover of history, is working more aggressively than any of his competitors to organize activists and volunteers ahead of the Jan. 21 primary, essentially pinning his candidacy on a state filled with Christian conservatives.

His chief rival, Mitt Romney, is approaching South Carolina tentatively. He invested huge sums in the state in the 2008 presidential race only to bail just days before the vote when it became clear he would lose big to Arizona Sen. John McCain. Many voters couldn’t overlook their skepticism of Romney’s Mormon faith and his reversals on some cultural issues.

The others in the 2012 race are treating South Carolina as an afterthought while they bank their candidacies on one of the two states that vote first, Iowa and New Hampshire.

Enter Gingrich, who’s enjoying a burst of momentum after a summer campaign meltdown.

“I do believe South Carolina will be the decisive primary,” the former House speaker from Georgia told Republicans who packed a theater in Newberry last week. “If we win here, I believe I will be the nominee.”

But victory in the state won’t come easy for the thrice-married Gingrich.

He has acknowledged having an extramarital affair, an issue that may turn off Christian conservatives who hold great sway in South Carolina. Gingrich, a recent convert to Catholicism, frequently makes a point of talking about his close partnership with third wife Callista.

He has advocated a “humane” approach to immigration that would let longtime residents work toward citizenship. Critics have labeled that as “amnesty” for millions of foreigners who are illegally in the United States, and that’s another potent issue in the state.

Winning in South Carolina would be even more difficult if he were to come in with a 0-2 record, losing in both Iowa and New Hampshire.

Perhaps for all those reasons, if not in spite of them, Gingrich is building the largest presidential organization in the state.