In a small Sioux City cafe, about a hundred people gather around Republican presidential candidate Rick Santorum, a Christian conservative and former senator.

The election rally begins with a prayer led by Protestant pastor Cary Gordon.

He speaks about the need to make the right choice in picking a Republican candidate who will face President Barack Obama in the November 6 presidential election.

Iowa Republicans will hold their caucuses to select their candidate on Tuesday.

The Reverend Gordon, who supports the candidacy of Santorum, talks about the need to send a message not only to America, but to the rest of world, about America's values.

In his television interviews and campaign speeches, Santorum has hammered home the message of rejection of gay marriage, and of abortion -- even in cases of rape.

Conservative Christians represent a key constituency in Iowa and other US states, where Republican primaries will be held until almost mid-year.

A report by the Institute for the Study of American Evangelicals estimated that the share of evangelical Christians in the United States is about 30 percent to 35 percent of the population, or around 100 million people.

Eileen Gordon, 62, the pastor's mother, also supports Santorum.

"I choose Rick Santorum," she told AFP. "He represents the moral values that are closest to my heart.

"We believe that God's natural law always supersedes man's law," she continued. "God's law is that a man and a woman produce a child. For that reason, we don't believe we can make a law that allows marriage between a man and a man."

Gerald Pallesen, 83, a World War II veteran, said he believes in "the sanctity of marriage".

"The person begins with conception, and they're going to use my tax dollars for abortion. I'm absolutely opposed to that," he said.

Santorum's efforts to court Christian conservatives appear to be paying off. The 53-year-old Pennsylvanian has pushed into third place with 15-percent support, just behind former Massachusetts governor Mitt Romney and Texas Representative Ron Paul, according to the latest Des Moines Register poll released Saturday.

Santorum's rise from single-digits in the poll was achieved, experts say, through meticulous door-to-door campaigning across the state.

"He's been to all 99 counties. He's been toiling away. In Iowa, that can pay off," said political scientist David Redlawsk.

Santorum is not the only candidate trying to lure the evangelical Christian voters to his side.

Texas Governor Rick Perry and Representative Michele Bachmann of Minnesota, both strong Christians, have also focused on religious issues during their campaign appearances.

Brad Zaun, an adviser to Bachmann, insists that she prays even onboard campaign buses.

"She does not back away from her beliefs," Zaun said.

Perry, a former Air Force pilot, told an audience recently how religion had helped him find the "right way" after he returned to civilian life from military service.

Jennifer Bowen, executive director of Right to Life, a non-profit organization in Iowa, declined to endorse any particular Republican candidate at this stage.

But she expressed her outright rejection of Obama.

"The candidate that we absolutely would not support is Barack Obama," Bowen said. "You know taxpayer money shouldn't be going to the abortion industry."