Influential evangelical Christian leaders endorsed Rick Santorum on Saturday for the Republican U.S. presidential nomination, in an attempt to strengthen him as the more conservative alternative to front-runner Mitt Romney.

At a weekend meeting at a ranch outside Houston, the group of 150 conservatives who had joined forces agreed on the third ballot to support the former Pennsylvania senator. Former House of Representatives Speaker Newt Gingrich came in second.

They had not been expected to reach agreement on one candidate since evangelical support has been splintered among Santorum, Gingrich and Texas Governor Rick Perry.

"There is clearly a united group here that is committed to see ... a true conservative elected to the White House," said Tony Perkins, leader of the Family Research Council and spokesman for the group.

The endorsement came one week before South Carolina votes on Jan. 21 in the Republican presidential primary. The Republicans are selecting a candidate to challenge President Barack Obama in the November election.

"It will have an impact in South Carolina and in shifting support to the consensus candidate which is Rick Santorum," Perkins said.

Santorum may need the help. A Reuters/Ipsos poll released on Saturday showed Romney with a huge lead in the state, with 37 percent of the vote. Santorum and libertarian Ron Paul were tied for second at 16 percent. 

Santorum will be looking to follow up with the religious groups to give him a push in the last week of campaigning in the state and into the next primary vote in Florida.

"We've got obviously great strength and support not just now here in South Carolina but lots of money has come in. We've raised over $3 million in a week," he said in Mount Pleasant, South Carolina.

Conservatives are seeking a viable alternative to Romney, who won the first two nomination contests in Iowa and New Hampshire and now leads the polls in South Carolina.

In the 2008 election, about 60 percent of the voters in South Carolina described themselves as evangelical Christians. Santorum is a Roman Catholic and father of seven who strongly opposes abortion and gay rights.

Townes Moore, a student at The Citadel military college in Charleston, South Carolina, campaigned for Santorum in Iowa, where the former Pennsylvania senator picked up much of the conservative vote to finish a surprising eight votes behind Romney in the caucuses.

"He is my role model," Moore said of Santorum, during a break from making calls at a phone bank in Mount Pleasant. "He is a devout Catholic and puts God at the center of his life."

Despite Romney's front-runner status, many conservatives mistrust him because of his record in relatively liberal Massachusetts, where he once supported abortion rights.

"Not a lot of time was spent on Mitt Romney," Perkins said, noting it was not a surprise he was not favored by the evangelicals. "It was more about the positive. How to get America back on the right road. How to get America great again."

The religious leaders debated and prayed over which candidate to pick, Perkins said, and chose the person they believed had the best social conservative and economic policies and was most likely to defeat Democrat Obama in the Nov. 6 election.

Some disputed the notion there was a complete consensus after the Christian leaders' meeting. Rick Tyler, a Gingrich aide and senior adviser to the "super" political action committee that has spent millions of dollars in advertising for him, said several heavy-hitting evangelical leaders still planned to endorse Gingrich.

"Endorsing Rick only serves to help Romney who has a terrible record on the issues evangelicals care about," said Tyler, who was not at the meeting but spoke to people who were.

Gingrich's campaign has begun airing TV ads in South Carolina that call Romney "pro-abortion," and charge that Romney - who says he now opposes the procedure - cannot be trusted to be reliably anti-abortion. In response, Romney began running a radio ad touting his anti-abortion views.

Perkins said all factors were taken into account at the Texas meeting and that Romney's Mormon religion "wasn't even discussed."