By Patricia Zengerle

WASHINGTON (Reuters) - In October, Texas megachurch pastor Robert Jeffress ignited a firestorm by calling Mormonism a "cult" and telling Republicans not to vote for Mitt Romney because the presidential aspirant was not a "competent Christian."

Today, Jeffress is endorsing Romney.

He is just one of many evangelical Christian leaders putting aside their suspicion of the former Massachusetts governor, a Mormon, to support him against President Barack Obama ahead of this November's election.

Other leading Christian backers include famous televangelist Joel Osteen, who gave Romney a nod on CNN last week when he said that while he does not see the Mormon faith as "traditional Christianity," he believes Romney subscribes to the Christian faith.

Many ordinary evangelicals have not come out for Romney. But some of their leaders quickly shifted away from previous support of former Republican presidential hopefuls like Rick Santorum and Newt Gingrich as Romney to become the likely Republican nominee.

"There are probably two primary reasons for it. ... The first is just an interest in getting a Republican in the White House," said Mark DeMoss, a senior Romney adviser who has served as a campaign ambassador to evangelical groups.

"The second is evangelicals, not unlike anybody else you could name, are interested in influence in government. And it's harder to have influence if you aren't part of the journey to winning," he said.

To be sure, there are holdouts among evangelical leaders, such as Gary Bauer, president of the conservative group American Values who ran for president as a Republican himself in 2000, backed Santorum and has not endorsed Romney.

Romney has nearly wrapped up the Republican nomination without the help of the religious right, raising the alarm among evangelical leaders who worry they might lose the role of Republican kingmakers they have held for 30 years.

Two leading anti-abortion groups - National Right to Life and the Susan B. Anthony List - declared support for Romney only two days after his main rival Rick Santorum dropped out of the Republican race on April 10.

The Susan B. Anthony List had campaigned for Santorum. But it now plans to spend millions of dollars on Romney's behalf, even though he is the only major Republican primary candidate not to sign the group's anti-abortion pledge.

Romney, in turn, will need Christian groups' votes - and organizational heft - with polls pointing toward a close contest in November. He can get them if he keeps to a socially conservative message, and does not take their vote for granted, strategists said.

In a push to win over evangelicals, Romney will speak at the May 12 commencement of Liberty University, an evangelical school in Virginia. It may be Romney's biggest audience until the Republican nominating convention in August.

"Evangelicals, when ... you have basically approached them and done the right due diligence, they tend to be a little bit like a 'Get out the vote' unit of your own," Republican strategist Ford O'Connell said. "It's not just get out the vote, it's spreading the word."

'CULTIST'

But it won't be easy for Romney.

Liberty University's Facebook page was so overloaded with angry comments - many labeling Romney "a cultist" - that the school removed the announcement of Romney's appearance.

"One of my friends is a Mormon and let me tell you they do NOT follow Jesus or the Bible as we do," one comment read.

There is still deep distrust of Romney among grassroots Christians, said Steve Deace, an evangelical Christian who hosts a nationally syndicated radio talk show.

"It will take the audible voice of God to get me to vote for Mitt Romney. I just know too much about the man's deplorable record and he's unrepentant about it to this day," he said.

A Pew poll late last year found that nearly two out of three white evangelicals do not believe Mormonism is a form of Christianity. Exit polls showed that Romney drew less than one-third of the evangelical vote in the primaries this year, although evangelicals represented half of all ballots cast.

A New York Times/CBS News poll found this month that 72 percent of white evangelicals would support Romney over Obama. But only 27 percent said they would enthusiastically support him, versus 42 percent of other Republican primary voters who said they would.

Some conservatives say they are not troubled by Romney's religion. They worry, instead, that he is a closet liberal because of his support for abortion rights and healthcare reform while running for the U.S. Senate and serving as governor in heavily Democratic Massachusetts.

"Trust is a big factor," said Gary Marx, executive director of the socially conservative Faith and Freedom Coalition.

"Romney didn't come from a (typical) conservative background, and doesn't share their faith, but the conservative background is far more important," said Marx, Romney's conservative outreach director during his failed 2008 nomination race.

Romney now opposes abortion rights and says he believes the Roe v. Wade decision allowing them should be reversed.

Conciliatory statements toward Mormonism from someone like the youthful Osteen, whose Houston church seats 16,000 worshippers, are likely to help Romney.

Even Jeffress said Mormons and Christians, though different, share many values, while Obama "embraces non-Biblical principles" such as support for abortion rights.

And partnership with Romney benefits the evangelicals.

"They are trying very hard to make sure that the Mormon faith is not going to be an obstacle going forward," said Villanova University professor Catherine Wilson, who studies politics and religion. "That's where you see the leaders coming out in favor of him, and how that is going to be tremendously helpful for the evangelicals."

"They need to be relevant too," she said.

(Editing by Alistair Bell and Philip Barbara)