"What evangelicals are saying is: We don't know what this guy believes," Scheffler said. "Does he have any public policy philosophy other than wanting to be elected president?"
Yet numerous polls and anti-Mormon statements suggest that deeper disagreements rooted in core elements of Christian theology are also in play.
A prominent Texas pastor (and Rick Perry supporter) has called Mormonism a non-Christian "cult." A Florida pastor says a vote for Romney is "a vote for Satan." The associate publisher of a leading evangelical magazine said a Romney presidency would "normalize the false teachings of Mormonism." A former staffer for Newt Gingrich's campaign said thousands of evangelical pastors stand ready to "expose the cult of Mormon."
Romney has acknowledged that his lifelong membership in the Utah-based Church of Jesus Christ of Latter-day Saints will cost him some votes. He told The New York Times last month that "most people don't decide who they're going to vote for based on the religion that they happen to be a member of. But there will be some for who that's an issue, and I won't get those votes in some cases."
The number could be as high as 15 percent among white evangelicals, according to a November poll by the nonpartisan Pew Forum on Religion & Public Life. A mid-December Salt Lake Tribune poll showed 25 percent of white evangelicals were uncomfortable voting for a Mormon presidential candidate.
That may not prevent Romney from winning the GOP nomination, but it could mean that millions of evangelicals stay home during the general election.
"Evangelicals have come to regard the presidency as a spiritually potent office," said Mark Silk, an expert on religion and politics at Trinity College in Hartford, Conn. "And the idea of electing someone who will use it on behalf of a religion they consider beyond the pale really bothers them."
All of which begs the question: Why does Mormonism make some evangelicals uneasy?
"At root, this is a theological argument," said Patrick Mason, a professor of Mormon studies at Claremont Graduate University in Southern California.
Among the disputes are the nature of God, the doctrine of the Trinity and the acceptance of revelations and books beyond the Christian Bible.
"For the people on the inside of these kinds of discussions, these are not just matters of life and death but of salvation. There is nothing more important for them than having a proper relation to God and idea of who Jesus is," said Mason, author of The Mormon Menace: Violence and Anti-Mormonism in the Postbellum South.
In a sense, Mormons and mainstream Christians have been at odds for nearly 200 years.
Mormonism's founding prophet, Joseph Smith, said God told him that every existing church and creed was "corrupt" and "wrong." Drawing on personal revelations published in the Book of Mormon and other texts Smith set out to restore Christ's church.
Smith preached fairly orthodox Christian theology at first, but "became increasingly radical, breaking more and more from standard Christianity with every year that he lived," said Craig Blomberg, a professor at Denver Seminary who has been active in evangelical-Mormon dialogue.
A sermon Smith preached three months before his death in 1844 planted the seeds for Mormonism's biggest break with traditional Christianity, according to scholars. In it, Smith preached that God was once a flesh-and-blood man who had attained godhood. Likewise, Smith taught, humans could advance to God-like status in heaven.
"It has become important for traditional Christians to maintain an unbridgeable creature-Creator chasm," said Robert Millet, emeritus dean of religious education at Mormon-owed Brigham Young University in Provo.
"For Latter-day Saints, God and man are the same species. God has substance he is not just a force or power. He is an exalted, glorified man, and one of the purposes of the gospel is to help us become what he is."
The idea of humans becoming gods runs counter to mainstream Christianity, said Richard Mouw, president of the evangelical Fuller Theological Seminary in Pasadena, Calif. Confusing the two has traditionally been considered blasphemous, he said.
However, the Mormon idea does approach the Eastern Orthodox Christian notion of "theosis," or partaking in the divine energies of God, said Mouw, a 20-year veteran of Mormon-evangelical dialogue.
The God-as-exalted man doctrine has profound effects on other areas of Mormon theology, according to scholars. For example, Mormons believe that God has a celestial wife, to whom Jesus was born in a premortal existence.
"We believe that Jesus and all humanity had a life before this life," Millet said, "and in that world, Christ was the eldest Jesus was our elder brother."
Thus, Jesus is a step below God on the stairway to heaven and not an equal member of the Trinity.
Traditional Christianity holds that God the Father, Jesus and the Holy Spirit coexist and share one substance. Mormons "deny the [doctrine of the] Trinity and that's huge," said Mouw.
But for all the theological fissures between Mormonism and evangelicals, some scholars say they have discovered a fair amount of common ground through dialogue.
"We are so close in some respects that when we differ, it can lead to inflammatory conversations," Blomberg said. "It's like a sibling rivalry."
The Salt Lake Tribune contributed to this story.