By John Whitesides

WASHINGTON (Reuters) - Two prominent Iowa religious conservative leaders endorsed Republican Rick Santorum on Tuesday, bolstering his longshot presidential candidacy and dealing a blow to front-runner Newt Gingrich two weeks before the state's kickoff nominating contest.

Bob Vander Plaats, head of the influential Christian group Family Leader and Iowa Family Policy Center head Chuck Hurley backed Santorum after the Family Leader's board of directors was unable to reach agreement on a broader endorsement.

"I believe Rick Santorum comes from us," Vander Plaats said of Santorum, who has touted his born-again Christian faith on the campaign trail but is mired in single digits in most state polls. "He's one of us."

Vander Plaats and Hurley said they would help Santorum in whatever way they can and expressed hope Christian conservatives could rally around the former U.S. senator from Pennsylvania in the waning days of the Iowa campaign.

The Family Leader's failure to reach a group endorsement was indicative of the difficulty Iowa's religious conservatives, a crucial voting bloc in the state, have had in reaching consensus ahead of the January 3 caucus.

It dealt a blow to Gingrich, the former speaker of the House of Representatives who has been sinking in polls over the last week and hoped an endorsement would inject new momentum in his campaign.

The Family Leader had been torn between endorsing Gingrich, Santorum, Congresswoman Michele Bachmann and Texas Governor Rick Perry as it searched for a faith candidate who could beat President Barack Obama in 2012.

Christian conservatives fear a split vote would open the door in Iowa for Republican Mitt Romney, the former Massachusetts governor who some conservatives distrust because of his past support for abortion rights.

"The fear would be a fragmented vote," Vander Plaats said, expressing hope some of the other contenders would drop out of the race before the caucus vote and back Santorum.

In 2008, six of every 10 voters who participated in Iowa's Republican contest said they were evangelical or born-again Christians, and they helped propel Mike Huckabee, a Baptist minister and former Arkansas governor, to a 2008 win.

Turnout among Christian conservatives in Iowa is likely to be lower this year given the uncertainty about which candidate to support, but Vander Plaats said Santorum was capable of catching fire.

"I think Rick Santorum has a chance to deliver on caucus night," he said. "I believe he is ready for a January 3 surprise."

A Santorum campaign official in Iowa, Jamie Johnson, said the endorsements would be a huge boost to his effort.

"It will help consolidate conservatives around Rick Santorum," he said.

The Family Leader group, which includes many social conservative voters and evangelical pastors in Iowa, had been prominent in the 2012 race with a presidential forum and a marriage pledge meant to get the candidates on the record on same-sex marriage.

Santorum, Bachmann and Perry signed the group's marriage vow, which calls for a constitutional amendment banning same-sex marriage, personal fidelity to one's spouse and the appointment of "faithful constitutionalists" to the federal bench.

Gingrich, whose two divorces have concerned conservatives, did not sign the pledge but promised to remain faithful to his third wife in a detailed letter recently sent to The Family Leader.

Santorum also has won the endorsement of evangelical pastor Cary Gordon of Sioux City, who helped get three Iowa Supreme Court justices voted off the bench for their role in allowing gay couples to marry in the state.

(Editing by Eric Walsh)