By Dan Gilgoff, CNN.com Religion Editor

Tampa, Florida (CNN) – The task of selling a Mormon presidential candidate to evangelical America has fallen to a public relations man who’s not even getting paid for what may be the toughest sales job of his career.

For six years, Mark DeMoss has served as Mitt Romney’s unofficial evangelical ambassador, making the case that born-again Christians should help elect the first Mormon to the White House.

It has often been a lonely job.

During this year’s primaries, DeMoss found himself addressing audiences of evangelical leaders in which nearly everyone was rooting for another candidate: Rick Santorum, Newt Gingrich, Rick Perry – anybody but Romney.

“It would have been tough for anyone other than Mark,” says Richard Land, the public policy chief for the Southern Baptist Convention, remembering how DeMoss performed in one hostile setting last January. “The audience was stacked for Santorum and Gingrich.

“But he has a lot of street cred with evangelicals,” Land says of DeMoss. “He understands us because he’s one of us. So he did great.”

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Now that Romney has outlasted the other candidates to become the Republican nominee for president, DeMoss is using that street cred to help the candidate close the deal with evangelical voters in the weeks before Election Day.

It’s unclear whether he will succeed.

Polls show that although most evangelicals have come around to Romney, there’s a sizable chunk who have not. With those voters making up a huge part of the GOP base in swing states like Ohio, Iowa and Virginia, whether DeMoss’ gambit works could mean the difference between an Obama or a Romney White House.

For DeMoss, who is officially a senior adviser to the Romney campaign, the stakes of his work go well beyond electoral politics. He’s trying to open the American evangelical mind.

“I took this on to tackle prejudicial attitudes,” DeMoss says, explaining how he approached Romney about running for president in 2006, convinced that the then-Massachusetts governor was the most qualified man for the presidency that he’d ever seen.

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“I discussed it with Romney the first time we met,” he continues, sitting in his room at the elegant Vinoy Resort and Golf Club in St. Petersburg, his home during the convention. “It bothered me that some evangelicals said they couldn’t support a Mormon for president. As a public relations guy, I wanted to change that mindset.”

Which is why DeMoss was in front of the North Carolina delegation at the convention Monday morning, arguing that it’s unfair for some Republicans to insist on a presidential nominee with whom they agree about everything.

“My advice to those folks is perhaps you should run yourself the next time,” DeMoss told the evangelical-heavy delegation in a Hilton Hotel ballroom, still abuzz about a powerhouse speech that New Jersey Gov. Chris Christie had delivered a few minutes earlier.

“My wife and I have been married for 24 years,” DeMoss said, “and I don’t agree with her on everything.”

Looking professorial in tortoiseshell glasses, a blue blazer and a white polo shirt tucked into khakis, DeMoss never mentioned religion or Romney’s Mormonism.

But you could tell it was a big part of what he was talking about.

It’s why he told the delegation that he has prayed with Mitt Romney and shared scripture with him and has even sought parenting advice from Romney and his wife, Ann.

After years of traveling and visiting with the Romney, DeMoss told the crowd, “I trust his values - for I’m fully convinced they mirror my own.”

It might not sound like much, but an evangelical Christian vouching for a Mormon’s values in front of ballroom full of fellow believers can be a powerful thing.

At least that’s the hope.

Lessons from the Moral Majority

DeMoss developed an appreciation for Mormons from a somewhat unlikely source: the evangelical giant Jerry Falwell.

He enrolled at Liberty University, Falwell’s school, in 1980, the year after his father died of a heart attack. Falwell, a fundamentalist preacher, would become like a second father.

DeMoss’s dad had been friends with Falwell – DeMoss says it’s unclear if the insurance marketing company his father founded, National Liberty Corp., helped give Liberty University its name – and Mark found work in Falwell’s office after graduation.

By the time he was 23, DeMoss was serving as Falwell’s chief of staff and spokesman, helping his boss run a growing evangelical empire that included the Lynchburg, Virginia, university and a new organization Falwell had helped found: the Moral Majority.

The organization aimed to bring evangelicals back into the political fold, after millions of them had spent decades sitting out elections, convinced that politics were a dirty, ungodly business.

“We traveled the country, challenging pastors to get involved. He outworked staff that were half his age” DeMoss says of Falwell, who died at 73 in 2007.

Falwell taught him how political organizing works, from the grassroots to the very top. He took him to meetings with President Ronald Reagan, whom the Moral Majority had helped elect, and President George H.W. Bush.

Among the most important lessons Falwell taught, DeMoss says, is that politics is the art of the possible.

“You’d be hard-pressed to find someone who was more politically conservative than Falwell, but he became increasingly pragmatic,” DeMoss says, eating blueberries from a plastic cup in his hotel room. “He was more practical and open-minded than a lot of people saw.”

As he waged crusades against abortion and for prayer in schools, Falwell proudly linked arms with non-evangelicals. While others in the burgeoning Christian Right wanted to organize only among their own flocks, the Moral Majority chief pushed an idea called co-belligerency: people of different religious backgrounds setting aside theological differences to pursue common political goals.

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“A lot of people forget this or didn’t know it to begin with, but the Moral Majority was a coalition of evangelicals, Catholics, Jews and Mormons,” DeMoss says. “It was not an evangelical organization.”

Mormons consider themselves to be Christians, but some evangelicals and other traditional Christians disagree. While Mormons treat the Bible as Scripture, they also consider the Book of Mormon to be a holy book

There are other big differences between Mormonism and traditional Christianity, including the Mormon belief that the modern prophets of the Church of Jesus Christ of Latter-day Saints can receive revelations from God. Traditional Christians believe that the period for such revelations is over.

But Falwell’s insistence on coalition building with Mormons and others stuck with DeMoss long after he left the Lynchburg in 1991 to start his own Christian PR firm in Atlanta.

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The firm, called the DeMoss Group, took Falwell as its first client and quickly added business from big Christian groups like Chuck Colson’s Prison Ministries, Franklin Graham’s Samaritan’s Purse and Christian Crusade for Christ.

More than 20 years later, those groups are still with DeMoss.

“What makes Mark different than a lot other guys in this business is that he’s independently wealthy,” says Graham, who notes that DeMoss’ wife, April, is also from a family that started an insurance company.

“I find him to shoot straight because he’s not trying to keep my business,” Graham says. “I think he’s more concerned with trying to be honest. He will bill you and then at the same time he’ll make a big contribution back to you.”

Though DeMoss has kept his work for Romney, which is unpaid, separate from the DeMoss Group, the relationships he built over decades through his PR work are key to selling Romney to evangelical leaders.

Graham had never met Romney before DeMoss arranged for 15 conservative Christian leaders to visit Romney’s Massachusetts home in 2007, when he was preparing to make his first run for president.

As the leaders took turns introducing themselves, many volunteered that they had traveled to Romney’s home mostly because DeMoss had asked them.

By the end of the meeting, Romney had made some new friends.

“Sometimes on TV someone can appear one way but when you meet them face to face you see the personal side of him,” says Graham, recalling the meeting. “After I met Governor Romney I liked him very much and even more l liked his wife and his marriage and his commitment to family.”

As for theological issues that interested some of the evangelicals, Graham says Romney “answered those questions extremely well.”

Since then, DeMoss has helped evangelical leaders not only become more comfortable with the idea of a Mormon in the White House but also with Romney’s evolving position on issues like gay marriage and abortion.

“He’s absolutely trusted as a pro-life person,” Land says of DeMoss. “When he says Governor Romney is pro-life, that means something. That helps.”

Land is among the many evangelical leaders who use DeMoss to relay concerns or advice to the governor.

“Mark’s a trusted negotiator,” says Land, who had dinner with Romney and DeMoss last year.

Though Romney’s 2008 campaign was unsuccessful, DeMoss counted it as a victory that no major evangelical figure came out against him over his faith, even if few publicly endorsed his campaign.

Four years later, there still aren’t many prominent evangelicals who’ve come out publicly for Romney.

And there are questions about where Romney stands with rank-and-file evangelicals. A recent Pew poll found that, while most white evangelicals support Romney, a quarter are uncomfortable with his religion. Just one in five in that group are strongly pro-Romney.

Ten weeks before Election Day, it’s not where a Republican nominee wants a key part of his base to be.

Visiting Salt Lake

DeMoss’ case for why evangelicals can enthusiastically support a Mormon candidate echo Falwell’s arguments about why evangelicals and Mormons should be political allies.

It goes like this: If evangelicals are OK with seeing a Mormon doctor or flying with a Mormon pilot, DeMoss reasons, shouldn’t they be OK with a Mormon president? We’re electing a commander-in-chief, not a pastor-in-chief, right?

Plus, fixing the national economy – the No.1 issue in this election – doesn’t really have anything to do with religion.

In fact, DeMoss was drawn to Romney because of the candidate’s unusual breadth of experience as a businessman, governor and Olympics Committee chief with dual degrees from Harvard.

“On a personal level and a spiritual level, I might care a great deal about what somebody believes doctrinally,” he tells NPR during a phone interview from his room at the Vinroy. “In the case of presidential election, I don’t care.”

After hanging up, DeMoss stays on that point: “I hope I’ve shifted a conversation about the religion of a candidate to one about the values of a candidate.”

DeMoss says that voting on the basis of a candidate’s faith is dangerous and inane. He notes that three of the most successful politicians from his own denomination, the Southern Baptist Convention, are all Democrats whom many evangelical loathe: Jimmy Carter, Bill Clinton and Al Gore.

Since January, DeMoss has spent about half his time making such arguments, stressing to clients that the work is not official firm business. Still, he suspects that some potential clients have skipped signing up with the DeMoss Group because its founder and president is pushing a Mormon candidate.

April, his wife, who’s checking her iPhone on the bed of DeMoss’ hotel room, says they’ve lost a few friends over Romney, too. But they’ve also made new Mormons friends, and have developed a deep appreciation for the Mormon faith.

On the van to the hotel to address the North Carolina delegation, Mark and April trade stories with their Mormon driver, a convention staffer, about their respective visits to Salt Lake City, where the Church of Jesus Christ of Latter-day Saints is headquartered.

Later, DeMoss talks about being turned off by the evangelical street preachers he’d seen on the street corners there, preaching to Mormons in town for the church’s annual general assembly. How could such evangelizers hope to convert anybody in the 30 or so seconds it takes to wait for the light to change?

For DeMoss, the episode represents a civility deficit when it comes to the evangelical treatment of Mormons. He sees his work with Romney partly as a corrective.

Whether DeMoss can help soften the evangelical line toward Mormons is an open question. So is whether he can get enough of his brethren to go a giant step further and vote for a Mormon for president.