CHARLESTON, S.C. — Time is running out for anyone to stop Mitt Romney from winning the South Carolina Republican primary Saturday, perhaps the last chance to keep him from running away with the party’s presidential nomination.

Newt Gingrich hopes that his aggressive, crowd-pleasing debate performance Monday night — always the oxygen for his campaign — will feed a surge of last-minute support.

Rick Santorum is hoping that the late backing of national social conservatives will turn into a grass-roots uprising for him in the Palmetto State.

But Romney leads polls by double-digit margins. And he’s using his well-financed campaign to build a firewall in the state’s most conservative counties to guard against voters surging late to a challenger, as they did to Mike Huckabee in 2008.

The likely result? The former Massachusetts governor could easily see his lead erode, but his competitors remain divided, they still face an onslaught of Romney campaign tools, and they may not be able to deny him a clear win.

“There’s no way that the opposition against Romney can form up,” said David Woodard, a Clemson University political scientist who’s also a Republican consultant.

The former Massachusetts governor leads with the support of about 32 percent of likely South Carolina voters, according to an average of recent public surveys by RealClearPolitics.com.

Gingrich, a former speaker of the House of Representatives, has about 22 percent. Santorum, a former senator from Pennsylvania, has about 14 percent, as does U.S. Rep. Ron Paul of Texas. Gov. Rick Perry trails far behind in last place. All those polls were taken before Monday night’s debate, at which Romney came under attack and sometimes was shaken.

Gingrich delivered a commanding performance, at one point bringing the Republican audience to its feet cheering with his defiant statement that America kills its enemies.

Eager to build on the momentum, Gingrich pressed South Carolina voters Tuesday to rally behind him as the only one who can stop Romney.

“If you vote for Sen. Santorum, in effect you’re functionally voting for Gov. Romney to be the nominee, because he’s not going to beat him,” Gingrich said in Florence. “The only way you can stop Gov. Romney, for all practical purposes, is to vote for Newt Gingrich. … It’s a mathematical fact now.”

Santorum is pressing hard as well, looking for social conservative support to grow since a gathering of national movement leaders decided Saturday to back him.

In 2008, 60 percent of the state’s voters called themselves evangelicals or born-again Christians, and they broke for Huckabee, a former Baptist preacher, although former Tennessee Sen. Fred Thompson cut into Huckabee’s total. That didn’t leave enough for Huckabee to overcome John McCain, who won the state and eventually the nomination.

Neither Gingrich nor Santorum has found a way to steal the other’s votes.

“Until one of those two figures out how to get votes from the other, they’re still going to divide the anti-Romney votes,” said Danielle Vinson, a political scientist at Furman University in Greenville, S.C.

The most likely place to watch for a late surge is in South Carolina’s socially conservative “Upstate,” a Bible belt around Greenville and Spartanburg.

Romney or his allies are working overtime there to shore up his support while trying to stop any movement to Santorum, deluging Upstate voters with mail and automated calls.

Some calls stress Romney’s conservative bona fides, such as one from a gun owner attesting to his support for the Second Amendment right to bear arms. Another is from a Massachusetts abortion opponent assuring skeptics that Romney is solidly opposed to abortion.

Still another refers to popular Sen. Jim DeMint, R-S.C., saying he thinks that Romney will win the state. That’s true, but the call apparently doesn’t mention that DeMint refused to endorse Romney this time, as he did four years ago.

Others accuse Santorum of voting to allow felons the right to vote, something driven home by TV ads aired by a pro-Romney independent political action committee. Santorum called it a smear campaign Tuesday, and he said he’d never supported allowing felons to vote while they were still in prison.

The ads, calls and mail Upstate may tamp down any anti-Romney uprising, Woodard said. Thus, if more pastors decide to speak out for Santorum in the campaign’s closing days, they may find themselves already crowded out.

“Romney has neutered the opposition,” Woodard said. “So there are some pastors who like Santorum. But their contact (with voters) will be the 13th after 12 contacts from Romney saying how wonderful he is. It can’t punch through.”