Mike Huckabee burst to national attention in Iowa late last year as the wise-cracking, guitar-strumming, former Baptist preacher who delivered his socially conservative message in such an amiable fashion that even many moderates were smitten.

Over recent days, however, the nice-guy image has slipped as he battles to maintain momentum from his Iowa victory by winning Saturday’s South Carolina primary.

Polls show him trailing behind John McCain, the Arizona senator, and defeat would raise questions about the sustainability of his low-budget campaign heading into a series of expensive, big-state primaries over the next month.

As the first Bible Belt state to vote, South Carolina was supposed to be friendly territory for the former Arkansas governor.

But he faces competition for conservative voters from Fred Thompson, another native southerner, who is making a last ditch effort to revive his stalled campaign in the state.

The former Tennessee senator and actor has gone on the attack against Mr Huckabee for raising taxes and spending and failing to crack down on illegal immigrants in Arkansas.

With Mr McCain expected to dominate among moderate Republicans and independents, Mr Huckabee has been forced to lurch to the right to shore up his evangelical base and prevent Mr Thompson outflanking him.

Nobody doubts Mr Huckabee’s commitment to social conservative causes, such as opposition to abortion and gay marriage, so he has focused instead on hardening his position on another hot button conservative issue: illegal immigration.

As recently as November, he was viewed as a moderate on immigration, having granted college scholarships to the children of illegal migrants in Arkansas.

But over the past month he has shifted to the right of the Republican field on the issue and this week became the first candidate to sign a pledge to bar illegal immigrants from citizenship and deport those caught in the US without documentation.

He took another step to the right on Thursday by declining to condemn the flying of the Confederate flag – a cherished symbol of southern heritage for some white conservatives but a divisive emblem of slavery and segregation for African-Americans.

”You don’t like people from outside the state coming in and telling you what to do with your flag,” he told supporters in Myrtle Beach. ”If somebody came to Arkansas and told us what to do with our flag, we’d tell them what to do with the pole.”

As well as buttressing him against Mr Thompson, his newly hard-edged approach is also aimed at Mr McCain’s weak spots.

The senator was a vocal supporter of failed legislation last year that would have created a path to citizenship for illegal immigrants – a proposal that many conservatives condemned as “amnesty”. He is also an opponent of the Confederate flag being flown outside public buildings, having voiced regret for adopting an ambiguous stance on the issue during his 2000 presidential campaign.

Mr McCain lost to George W. Bush in South Carolina eight years ago in large part because he failed to win significant support among evangelical Christians and other right-wing voters. He has reached out to conservatives this time by stressing his opposition to abortion and shifting position on immigration to place greater emphasis on border controls. His strength on national security is also an asset in a state where more than 10 per cent of people are military veterans.

But many on the right – particularly the religious right – still view him with suspicion. Polls show Mr Huckabee leading in South Carolina among those who describe themselves as “very conservative” or “Born Again”.

Mr Thompson is tussling with Mitt Romney, the former governor of Massachusetts, for third place but almost certainly needs to finish first or second to keep his campaign alive after disappointing results in earlier states.

His southern drawl and laid-back style fell flat north of the Mason Dixon line but he has looked in his element in South Carolina and polls show him nudging upwards – mostly at the expense of Mr Huckabee.

“It’s sure good to be back among folks who don’t think I talk funny,” he told about 300 supporters packed into the Coach House restaurant in Simpsonville, as black waitresses maneuvered among the all-white crowd carrying large trays of pancakes and burgers.

“Some people think the way to win is to become more like the Democrats,” he said, prompting pantomime boos from the audience. “I could not disagree more.”

A further 100 people were locked outside, forced to peer through the restaurant windows to catch a glimpse of the candidate while listening to his stump speech through loud speakers. “We’re the only campaign that’s on the move,” said Gresham Barrett, a local Republican congressman who is backing Mr Thompson.

Every Republican questioned by the Financial Times gave a similar reason for supporting Mr Thompson: he is the most authentic conservative in the race, they said. “He’s the only candidate I can support without compromising my principles,” explained Jack Delvo, a private security officer.

Esther Wagner, a local volunteer for the Thompson campaign, said she respected Mr Huckabee’s Christian beliefs but did not trust him on economic issues and national security. “I’m not choosing a preacher, I’m choosing a president,” she said. Tina Reusch, an IT director, was even more scathing about Mr McCain. “He’s a liberal, plain and simple,” she said.

The danger for Thompson supporters is that their votes could split the conservative base and clear the way for a McCain victory.