It is the rare presidential candidate who comes to Idaho to raise money, but there was Governor Romney last month, packing more than 100 people, at up to $2,300 a head, into the Crystal Ballroom in Boise.

“Nearly every seat was filled. Just about everybody that’s anybody was there,” a former Idaho state legislator, Grant Ipsen, said. “I don’t think I’d ever attended another fund-raiser for a federal candidate in Idaho.”

There was no great mystery why Mr. Romney was in town. The former Massachusetts governor is a Mormon, as are about one-quarter of Idaho residents, including Mr. Ipsen and many others who turned out for the lunchtime event. The fund-raiser was bracketed by two others in the Mountain West: one in Las Vegas and another outside Phoenix. At both of those events, Mormons made up at least half the crowd, organizers said. Altogether, the two-day swing brought in well over $1 million for Mr. Romney.

As he vies for a place in the top tier of contenders for the Republican nomination, Mr. Romney is reaping enormous benefits from being part of a growing religion that has traditionally emphasized civic engagement and mutual support. Mormons are fueling his strong fund-raising operation, which this week reported raising $21 million, the most of any Republican candidate. And they are laying the foundation for a potent grass-roots network — including a cadre of young church members experienced in door-to-door missions who say they are looking forward to hitting the streets for him.

“When Mormons get mobilized, they’re like dry kindling. You drop a match and get impressive results quickly,” said University of Notre Dame political scientist David Campbell, who is Mormon. “It’s almost a unique group in the way in which it’s organized at the local level and the channels through which mobilization can occur.”

But the intensity of this support has a potential downside as Mr. Romney tries to establish an identity separate from a religion still regarded warily by many Americans — a quarter of whom, polls suggest, do not want a Mormon president.

Mr. Romney’s fellow Mormons also find themselves in a bind. In dozens of interviews, Mormons across the country said they are excited by Mr. Romney’s candidacy and eager to do what they can for him, just like members of other religious or ethnic groups with favorite-son candidates. Yet they are also hesitant to state their support too strongly, to avoid provoking anti-Mormon bias or violating church rules against politicking inside church walls.