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The Republican presidential candidates are keeping a close eye on South Carolina even as they prepare for the first-in-the-nation New Hampshire primary, with new polls showing the leaderboard shifting in the Palmetto State contest

Three polls Friday showed Mitt Romney reclaiming the lead from Newt Gingrich in South Carolina following the former Massachusetts governor's narrow win Tuesday in Iowa. Rick Santorum and Gingrich are now battling for second in South Carolina -- whose election results historically have been the most accurate gauge for who becomes the Republican presidential nominee.

"It's going to come down, as it always does, to South Carolina," Sen. John McCain, the 2008 GOP nominee, said Friday while on the stump with Romney in Conway, S.C. "If Mitt Romney wins here, he will be the next president of the United States."

McCain predicted Romney would win in New Hampshire, hardly a radical forecast at this point. Most polls show Romney well ahead in the state, though the race there is tightening a bit.

Although all the GOP candidates are competing in New Hampshire, even before the Iowa caucus results came in they were plotting for how to make inroads in South Carolina and prevent Romney from going three for three.

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Gingrich said in an interview Friday that one of Romney's rivals "will eventually emerge as the conservative alternative and will beat Romney."

Gingrich lost an earlier lead in South Carolina but he remains competitive there despite a fourth-place finish in Iowa. A Rasmussen Reports poll out Friday showed Romney with 27 percent support in South Carolina, followed by Santorum with 24 percent and Gingrich with 18 percent. An American Research Group poll in the state showed Romney with 31 percent, and Gingrich and Santorum each with 24 percent.

A CNN/Time poll later showed Romney with 37 percent, followed by Santorum at 19 percent and Gingrich at 18 percent.

The polls reflected a steady rise for Romney and an astonishing surge for Santorum following his near-tie with Romney in Iowa -- Rasmussen had him at 1 percent in South Carolina just two months ago.

The Rasmussen poll of 750 likely GOP primary voters was taken Thursday and had a margin of error of 4 percentage points. The ARG survey of 600 likely GOP voters was taken Wednesday and Thursday and also had a margin of error of 4 percentage points. The CNN/Time poll of 485 likely primary voters was taken Wednesday and Thursday, with a margin of error of 4.5 percentage points.

McCain and other Romney surrogates hammered at the candidate's rivals ahead of the New Hampshire and South Carolina contests. McCain criticized Gingrich and Santorum for supporting earmarks in Congress, telling voters, "My friends, earmarks are the gateway to corruption."

Romney's GOP rivals are likewise working overtime to cast as him to too timid and too moderate: They're urging Republicans to do themselves a favor and nominate a more conservative standard-bearer offering a sharper contrast to Obama.

"The only way Republicans lose is if we screw this up and nominate another moderate who has taken multiple positions on every major issue of our time," Santorum, a former Pennsylvania senator, told supporters in a fundraising appeal Friday.

Gingrich argued on ABC News that Romney can't win the nomination and said that even if he did, his performance against Obama in the general election campaign debates would be laughable.

Romney is heavily favored to win Tuesday's New Hampshire primary, so much so that he can afford to focus on South Carolina, where voters aren't due to cast primary ballots for another two weeks.

Romney kept his focus on Obama, telling his audience in Conway that the president's proposal to reduce the military and focus more on Asia was "inexcusable, unthinkable and it must be reversed."

Santorum, who pulled within a handful of votes to place just behind Romney in Iowa's caucuses, is likely to find a welcome audience among South Carolina conservatives, and so he remained in New Hampshire to try to maintain the momentum he earned from Iowa.

"Don't settle for less than America needs," Santorum asked those expected to vote in New Hampshire's first-in-the nation primary. Without saying so Thursday, he and the other candidates appeared to share a common objective -- hold down Romney's vote totals in New Hampshire, then knock him off stride in the first Southern primary.

Romney benefited handsomely from having several rivals split the vote in Iowa, where his winner's share was roughly 25 percent.

His allies were fully engaged in the tussle over which GOP candidate is the true conservative. On Friday, he showcased the endorsement of conservative leader Bay Buchanan, whose brother Pat won the New Hampshire primary in 1996. Bay Buchanan cast Romney as a "real conservative" who could get things done.

Restore Our Future, the pro-Romney super PAC that unloaded a barrage of negative TV ads on Gingrich in Iowa, planned to go after him again -- this time in print. The group announced Friday it had purchased full-page newspaper ads in New Hampshire and South Carolina tying the former House speaker to Obama.

"On issue after issue, Newt Gingrich and Barack Obama have so much in common, the right choice is to choose neither," the ad said, ticking through issues, including backing the federal bank bailout and favoring "amnesty" for illegal immigrants

Jon Huntsman, who bypassed Iowa to bet his campaign on a good finish in New Hampshire, was showing off an endorsement by The Boston Globe, Romney's hometown paper. It was the second time Massachusetts' largest newspaper had snubbed Romney ahead of the New Hampshire primary.

Also vying to emerge as Romney's chief rival were Texans Ron Paul and Rick Perry.

Perry, who finished fifth in Iowa, released a biographical ad in South Carolina that spokesman Ray Sullivan said shows his "perfect-for-South-Carolina status" as a conservative man of faith and a veteran.

Paul, who placed third in Iowa, was arriving in New Hampshire on Friday, in time to campaign and participate in a pair of weekend debates.

The Associated Press contributed to this report.