In theory, the evangelical political movement says that it is prepared to embrace Jews and even Muslims so long as they share the same common values of the religious right. In the case of a Mormon candidate, though, many evangelicals are not prepared to say that common values are enough. The reason seems to be the view among evangelicals that the substantive theological beliefs of Mormons are so radically different from their own as to constitute not a sect of Christianity but a Christian heresy, which would be worse than a different monotheistic faith like Judaism or Islam.

What began as a strategy of secrecy to avoid persecution has become over the course of the 20th century a strategy of minimizing discussion of the content of theology in order to avoid being treated as religious pariahs. As a result, Mormons have not developed a series of easily expressed and easily swallowed statements summarizing the content of their theology in ways that might arguably be accepted by mainline Protestants. To put it bluntly, the combination of secret mysteries and resistance in the face of oppression has made it increasingly difficult for Mormons to talk openly and successfully with outsiders about their religious beliefs.

The general pattern of Mormon history is one of growth leading to external pressure being brought to bear on the church. Internal resistance eventually gives way to change sanctioned by new revelation, followed in turn by new growth and success. This was the pattern not only for the abolition of polygamy but also for the extension in 1978 of the Mormon priesthood to black men. Mitt Romney's run for the presidency is the occasion for the latest round in this cycle, with cultural and religious skepticism representing the vector for outside pressure.

Precisely because Romney is such an impressive candidate, it may be a slap in Mormons' faces if he finds that he cannot garner the support of conservative values voters. If such voters prefer, say, a pro-choice Roman Catholic of questionable conservative credentials like Rudy Giuliani, the result may look like a public repudiation of Mormonism — from the very party to which Mormons have given their allegiance for the last half-century. The Mormon Church hierarchy may through continuing revelation and guidance respond by shifting its theology and practices even further in the direction of mainstream Christianity.

Voices within the LDS fold have for some time sought to minimize the authority of some of Joseph Smith's more creative and surprising theological messages, like the teaching that God and Jesus were once men. You could imagine Mormonism coming to look more like mainline Protestantism with the additional belief — not in principle incompatible with Protestant Scripture — that some of the lost tribes of Israel ended up in the Americas, where a few had a vision of Christ's appearance to them.

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Noah Feldman, a contributing writer to The New York Times Magazine, is a law professor at Harvard University and adjunct senior fellow at the Council on Foreign Relations.