COLUMBIA, South Carolina (Reuters) - A group of 150 influential Christian conservative leaders on Saturday backed former Pennsylvania Senator Rick Santorum as their choice for Republican U.S. presidential nominee.

The conservatives agreed to support Santorum on the third ballot in a meeting at a ranch outside Houston, Texas.

"What I did not think was possible appears to be possible," said Tony Perkins, leader of the Family Research Council and spokesman for the group.

The group had not been expected to reach agreement on one candidate since evangelical support was divided between Santorum, former U.S. House of Representatives Speaker Newt Gingrich and Texas Governor Rick Perry.

The backing gives Santorum a boost against Republican front-runner Mitt Romney, particularly in South Carolina which holds the next primary on January 21.

In the 2008 election, about 60 percent of the voters in South Carolina described themselves as evangelical Christians.

"There emerged a strong consensus around Rick Santorum," Perkins said in a telephone news conference after the meeting. "While there was not a lot of hope that we could reach consensus around one candidate, that was achieved."

(Reporting by Deborah Charles; Editing by Vicki Allen)