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 REXBURG, IDAHO

"America's Family Community." That's the motto of this Mormon college town, displayed on street-side monuments and in tall letters on the movie-theater marquee. Apparently, it's a formula for success. Rexburg thrives on a burst of construction and population growth. More than 30,000 residents occupy a grid of wide, orderly streets, amid vast potato fields that unfurl toward the majestic Teton Peaks. Plenty of Rexburg parents, following the Mormon prescription for big families, have six or seven children. One guy tells me his next-door neighbors have 13 children, and a family on the other side has 16. The newly expanded hospital maternity unit is already crowded with new babies. If Rexburg is any indication, Mormons are taking over the world.

They certainly run this town. An estimated 97 percent of the locals belong to the Church of Jesus Christ of Latter-day Saints -- making Rexburg possibly the most Mormon of all towns. The brilliant white-stone 57,000-square-foot Mormon temple, opened eight months ago, looms on a hilltop, glowing day and night; intense floodlights make Mormon temples the brightest objects in the Western nights. The college that sprawls beside the temple -- Brigham Young University-Idaho -- now boasts an annual enrollment of 21,000 students, more than double what it had eight years ago.

Mormon mores -- some written into local laws -- permeate the community. Rexburg has no real saloon and no supply of hard liquor; only four restaurants are licensed to serve beer or wine. There is only one coffee shop, and it keeps up with the meager caffeine demand by brewing each cup individually. When I cruise town on a pleasant Saturday night in mid-September, the hottest action comes down in a bowling alley: Balls crash down all 16 lanes while the spinning pins and the bowlers' teeth glow even whiter under the ultraviolet lighting.

But something louder and bigger draws me to Rexburg: the religious culture wars, which heat up every election season. The prophets who run the Mormon Church -- the church president, his two counselors and a dozen top apostles, based in the headquarters in Salt Lake City, Utah -- encourage all Mormons to be active in politics. The prophets are said to be relaying the word of God, and while they generally don't endorse candidates, they take stands on issues such as abortion and homosexuality. As a result, most Mormons vote very Republican. In the last presidential election, nearly 92 percent of the votes in Madison County (Rexburg is the county seat) went to George W. Bush -- securing Rexburg yet another title: the nation's most Republican town.

On the most critical issues, the Mormon prophets go all out, urging their followers to conduct targeted campaigns. That helps explain why, Thursday evenings in the downtown building of a health-products company owned by one of Idaho's richest Mormons, groups of Rexburg college students and townies get together. They're using the company's call center to make call after call to California voters, trying to persuade them to pass a ballot measure in the November election. It's titled Proposition 8 -- the California Marriage Protection Amendment -- and it aims to prevent gay and lesbian people from getting married in that state.

An eight-year battle led to Proposition 8. In 2000, with Mormon encouragement and campaign money, California voters passed a measure banning gay marriage. It blew up again last May, when the California Supreme Court justices narrowly ruled (four to three) that the ban violated the civil rights of gays and lesbians. The court likened it to the bans many states once had against interracial marriage, all of which were tossed out long ago. Now, Proposition 8 aims to overrule the California Supreme Court, by amending the state Constitution.

Many religious groups have jumped into the campaign; the Mormon Church takes the lead. In June, the church's top prophets commanded Mormons "to do all you can" to work for Proposition 8 and donate money to the campaign. Mormon leaders throughout California read the instructions to their congregations, which have more than 750,000 members. Word spread everywhere in the Mormon realm. In August, the prophets added pages of elaboration: "The Church has a single, undeviating standard of sexual morality: intimate relations are proper only between a husband and a wife united in bonds of matrimony. ... Any dilution of the traditional definition of marriage will further erode the already weakened stability of marriages and family generally ... with harmful consequences for society."

Mormon volunteers, additionally inspired by special TV broadcasts beamed from the headquarters into their churches, go door-to-door in California for Proposition 8. In other states, they run phone banks and do whatever they can. Their effort is strongest in the West, because there are more Mormons in this region than anywhere else. Chad Reiser, a leader of the BYU-Idaho College Republican Club, says the phone banks are not an official club activity, but "we do try to get as many people involved as possible. Proposition 8 is a moral issue" related to church doctrine -- "something we believe is important to all people."

Kim B. Clark, the president of BYU-Idaho and a pillar of Rexburg's Mormon establishment, receives me in his office on a sunny Tuesday morning. His windows look out on construction cranes erecting a huge events center that will have a 15,000-seat auditorium and 10 basketball courts. He talks of more university projects. He appears confident and wears a pinstripe charcoal suit and red-pattern necktie, with a well-thumbed 2,000-page book of scriptures within reach. He grew up a Western Mormon in Utah and Washington, earned a Ph.D. at Harvard University and ran the Harvard Business School. He left Harvard three years ago, because the top prophet invited him to shape this college. It was "like getting a call from Moses," Clark says.

When I ask Clark about his church's campaign against gay marriage in California, I note that some people consider it a violation of the U.S. Constitution and Bill of Rights -- which mandate the separation of church and state. He strongly disagrees. Those who would limit his church's work against homosexuality, he says, "cloak their arguments in other terms ... (such as) civil rights ... but their fundamental purpose is to destroy religion in our society."

He says these battles will occur more and more frequently: "We're seeing a change in the political environment and interest groups. ... It takes many different forms. It's not directed at any particular religion. It's driven by people who are against religion."

As over-the-top as that sounds, Proposition 8 is a Western showdown with national implications. Hollywood celebrities, civil-rights groups, dissident Mormons, mega-businesses such as Google, politicians and other interests around the country have also jumped in. Shortly, the election results on Proposition 8 will create a new landmark signifying how much influence religions can assert on our modern society. Behind all the fury, I find something unexpected in the Rexburg area. It seems to me that, despite appearances, the Mormon Church may be losing its grip.