It started with a prayer: "We thank you, Lord, for Rick Santorum." Then a local Republican senator addressed the crowd. "There have been many great leaders throughout history," he declared. "Obviously, the greatest was Jesus Christ..." But here, ladies and gentlemen, was another.

Rick Santorum strode to a lectern on the steps of Oklahoma's state capitol on Sunday, and addressed his people. In a speech lasting 45 minutes, and delivered without notes, he held forth on the core virtues at the centre of his campaign for the US Presidency: faith, family, and small government.

"You are the conservative lodestar," he told them. "You can send out a very strong message here. You can go out, and give us a win and, I guarantee you, we will go on past Super Tuesday. We will go on to Alabama and Mississippi, and win there, and this race will turn around, and we will go on and be the nominee."

It was vintage Santorum: unpolished, sometimes inelegant, and genuinely unapologetic in its pitch to the religious Right who dominate Republican discourse in "heartlands" such as Oklahoma, which is one of 10 States that go to the polls today, in the biggest single test of this Primary season.

He slammed "Obamacare", the President's healthcare reform, and the Wall Street bailouts. He wondered why the US government doesn't do more to encourage people to marry before having children. Then he ridiculed the "radical forces running Washington" for buying into the "hoax" of global warming. "C02, according to this administration, is toxic," he said. "Well, go tell that to a plant."

Aside from a small cabal of Occupy Wall Street protesters, the crowd lapped it up. As a staunch Roman Catholic, Santorum is a hero to evangelical voters who make up around 70 per cent of Republicans in Southern states. He currently boasts the support of more than 40 per cent of such voters, roughly twice the amount of any of his rivals.

"Rick Santorum has morals and values," said Danielle Capelen, an "army wife" who attended with her three children. "It's hard to explain exactly why, but there's a difference between him and the others. He's by far the most genuine guy out there; someone we can trust."

For a would-be leader of the free world, Mr Santorum's social positions put him towards the extreme end of the spectrum. He is opposed to not just abortion, but also contraception. He does not believe in evolution. He prefers home-schooling to public education. And he has likened homosexuality to "man-on-dog" sex.

In a recent speech, Mr Santorum claimed that the separation of church and state, a fundamental principle of US constitutional democracy, made him "want to throw up". This week, his campaign said he stood by a doomsday allegation, made to a religious audience in 2009, that: "Satan has set his sights on America."

But if recent polls are to be believed, voters such as Ms Capelen will nonetheless propel Santorum to an easy victory in Oklahoma, which is the most conservative state in the nation (of its 77 counties, 77 voted Republican in 2008). They have also given him a narrow lead in Tennessee, which votes today and, as things stand, are capable of pushing him over the top in such coming battlegrounds as Missouri, Mississippi, and Alabama.

With Newt Gingrich expected to carry his native Georgia today, Ohio too close to call, and only limited polling available in this Super Tuesday's other voting states of Alaska, North Dakota and Idaho, that could spell a tricky 24 hours for Mitt Romney, the longstanding front-runner who has so-far failed to close out the increasingly hostile race for the Republican nomination.

Mention Romney's name to supporters of Santorum, and you are met with furrowed brows. Unlike their man, a former senator with seven children, who wears his beliefs on his sleeve, Romney seems untrustworthy, they say, adding that he perhaps isn't a true conservative. "I vote with my gut," said Lewis Whitaker, a retiree at Sunday's rally. "In my gut, there's something wrong with Mitt."

When voters talk of "gut feelings," or "flip-flopping," they are often speaking in a code. In evangelical circles, Romney suffers from a major, often unspoken handicap: his Mormon faith. And in the so-called Bible Belt, discourse is dominated by the Southern Baptist Church, which regards Mormonism as a perversion of Christianity.

"For Baptists, Mormonism is no better than Islam. It's just wrong, it's just false, and dangerously so," says Charles Kimball, the director of University of Oklahoma's religious studies programme. "You get people giving all sorts of reasons why they don't like Romney, but if you look at where the opposition comes from, it is often evangelical."

Though they are legally prevented from actually endorsing candidates from the pulpit, many Southern Baptist ministers talk politics to their congregations, admits Todd Littleton, pastor of Snow Hill Baptist Church in Tuttle, just outside Oklahoma City.

"If we Baptists also suffer an inglorious characterisation that we are bigoted towards Mormons, it's probably because we've done something to merit that image," he adds. "You won't find people being what I would call militantly anti-Mormon. But those conversations probably do happen."

Only one Southern state has voted so far, and that was South Carolina, where support for Romney mysteriously vanished on polling day, handing a thumping victory to Newt Gingrich, at the time his nearest rival.

The "anyone but Mitt" role has now fallen to Santorum, and his pull on evangelical voters, particularly in the Bible Belt, may soon represent the only serious barrier between Romney and the nomination.

Super Tuesday: the states that vote

Candidates need to secure the backing of 1,144 delegates at the Republican convention in the summer in order to take on President Obama in November. Tonight, they'll fight it out for delegates across 10 states.