CNN / Peter Hamby

Greenville, South Carolina (CNN) -- For most of 2011, South Carolina was the forgotten stepchild of the Republican race.

As former Republican Gov. Mark Sanford put it, "Just a few weeks ago, most people I talked to wouldn't know where to find a bumper sticker."

Things have changed, and quickly.

Campaign ads are now inescapable. The mail pieces are beginning to hit. Phones are starting to ring.

The state is gearing up for what might be the decisive fight of the Republican primary race -- a showdown between Mitt Romney and a clutch of conservative challengers scrambling to derail the front-runner before the race heads to Florida and other expensive states where television ads trump handshakes.

"The other campaigns have decided Romney is the dragon they have to slay," said one well-connected Columbia insider who talks regularly with all of the presidential campaigns. "This is their last chance to stop Romney."

But the question that dogged conservatives in Iowa remains true and even more pressing in South Carolina: Who will be the one to stop him?

A CNN/TIME/ORC poll of likely South Carolina Republican voters taken after last week's Iowa caucuses showed Romney with a commanding lead over his nearest rival.

Romney had the support of 37% of Republicans, followed by Rick Santorum at 19%, Newt Gingrich at 18% and Texas Rep. Ron Paul at 12%.

Texas Gov. Rick Perry, who is staking his campaign on South Carolina, is lagging far behind at 5% but remains ahead of former Utah Gov. Jon Huntsman at 1%.

Perhaps more eye-popping than his wide overall lead, Romney outpaces his opponents in nearly every category and demographic. He bests his rivals among men, women, born-again Christians, tea party sympathizers, college graduates, low-income voters, city dwellers and rural Republicans.

But the most important number from the poll is 49%: That's the percentage of Republican voters who said they might change their minds before the January 21 primary.

With Romney looking like an all but certain victor in New Hampshire, the campaigns are already appealing to those ambivalent South Carolina voters with a wave of television ads and mailers -- many of them negative.

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A super PAC supporting Romney has already been airing ads for a week attacking Gingrich and Perry. A pro-Perry super PAC has been hammering Romney and Gingrich.

Gingrich has a spot attacking Romney. And a tough new ad from Paul calls Santorum a "corporate lobbyist and Washington politician."

Santorum, according to conversations with more than two dozen Republican operatives and activists here, is best positioned to emerge at the strongest Romney challenger in South Carolina.

While he and Gingrich were statistically tied in the CNN poll, Santorum's near-victory in the Iowa caucuses seems to have awakened a sleeping army of evangelicals and anti-abortion activists who have been searching for a candidate to get behind.

Charleston County Republican Party Chairwoman Lin Bennett told CNN she stopped by Santorum's Lowcountry headquarters in Mount Pleasant on Tuesday night, once it became clear Santorum would have a big night.

"It was so crowded, I could hardly get in the door," said Bennett, who is neutral in the Republican race.

She attributed the fresh enthusiasm, in part, to the groundwork Santorum began laying in the state beginning in 2009.

"He has been around here doing that eyeball-to-eyeball thing, like he was doing up in Iowa, the retail politics," Bennett said. "He has a ground game here that is ready to be set loose."

The Santorum campaign organized a show of grassroots muscle when he arrived in Greenville on Sunday, his first appearance in the state since his narrow-second place finish in Iowa.

Santorum picked up the support of conservative leader Gary Bauer after he touched down in South Carolina, along with endorsements from a trio of Republican leaders.

He also set up a series of closed-door meetings with South Carolina legislators, CNN has learned.

Santorum's main rival for the anti-Romney vote is Gingrich, and the two are dueling for support from the state's vibrant tea party movement.

Long before Gingrich surged in the polls in December, he had relocated staffers to South Carolina and began hailing the state as his "Southern firewall."

Gingrich then hired several tea party activists onto his Palmetto State team.

Gingrich and Santorum will address the South Carolina Tea Party Coalition Convention in Myrtle Beach on January 15-16.

Both Republicans appear to be concentrating their efforts on the heavily conservative Upstate, a region peppered with megachurches that delivered big margins to Mike Huckabee and Fred Thompson in the 2008 primary while John McCain racked up votes in the counties along the more socially moderate, military-friendly coast.

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But supporters of both Santorum and Gingrich fret that 2012 will be a repeat of 2008, when conservatives failed to unite behind a single candidate and allowed the more moderate McCain to escape South Carolina with a pivotal win.

"I would hope that the grassroots movement could coalesce around one of the anti-Romney candidates, but I am not sure that is going to happen," said Allen Olson, a Columbia tea party activist who is working for Gingrich.

Olson said either Gingrich or Santorum would have to drop out of the race soon, perhaps after South Carolina, if there is any hope of stopping Romney.

The divided conservative vote is one reason Romney is competing so aggressively in a state that handed him a dismal fourth place finish in 2008.

Apparently comfortable with his standing in New Hampshire, Romney took a 24-hour hiatus from the Granite State late last week and went to South Carolina to campaign with McCain and South Carolina Gov. Nikki Haley, two of his supporters.

"He's going to win in New Hampshire, and it's going to come down, my friends, as it always does, to South Carolina," McCain told voters at one of their rallies in Conway, near Myrtle Beach. "If Mitt Romney wins here, he will be the next president of the United States."

Romney is running a television ad statewide calling the need to slash government spending "a moral responsibility."

His campaign also recently sent mailers to thousands of South Carolina Republicans touting Romney's family values and faith.

Conservative activists fiercely opposed to the former Massachusetts governor have started to push back, circulating anti-Romney screeds and videos on Facebook.

A Massachusetts-based anti-Romney group called "MassResistance.org" recently e-mailed hundreds of S.C. activists YouTube links of Romney taking moderate positions on abortion and same-sex marriage during his 1994 Senate race against Ted Kennedy.

Romney's detractors in the state have groused about Haley, who was elected in 2010 on a wave of grassroots enthusiasm but whose approval ratings as governor have slipped after warring with Republican legislators in Columbia.

Romney advisers have pushed back on the criticisms. One Romney source shared internal campaign polling, taken on Wednesday and Thursday, that showed Haley with a 66% job approval rating among Republican voters.

There are several wild cards that could shift the landscape of the race in the nearly two-week stretch before January 21 contest, an open primary in which all registered voters can participate.

There are two upcoming debates: one in Myrtle Beach on January 16 and another in Charleston on January 19.

There is also the possibility that U.S. Rep. Tim Scott, an enormously popular figure among Republicans the state, might deliver a last-minute endorsement.

Scott has said he would like to throw his support behind a candidate and has hosted a series of presidential town halls with each of the contenders.

While U.S. Sen. Jim DeMint, the most coveted endorser in the state, has said he will not get behind a candidate, there are whispers among some Republicans that Scott's endorsement will be a proxy for DeMint's.

Finally, there is Perry.

As a Southerner and a veteran, Perry brings a cultural appeal to South Carolina that his rivals lack.

More importantly, the Texas governor has nothing to lose -- and he has anywhere between $1.4 and $3 million in the bank, according to Republican sources.

That's more than enough money is enough to saturate the state's media markets with television ads for two weeks.

"I get that we are the underdog and we are certainly not the establishment candidate, but South Carolina ain't going to let Iowa pick its nominee," said Katon Dawson, Perry's senior adviser in South Carolina.

After Perry flopped in Iowa in Tuesday's caucuses, Dawson immediately picked up his phone and dialed national reporters to boast about Perry's better chances in South Carolina.

He fed all of them the same line: "Iowa picks corn, New Hampshire picks presidential campaign pockets, and South Carolina picks Republican presidents."