ROCKY RIDGE, Utah — About 60 miles south of Salt Lake City, at the end of a long, winding, dirt road off the highway, a small cluster of houses sits at the foot of a desert mountain. The neighborhood landscape is littered with children's things — plastic playground equipment, abandoned toys — and the backyards are covered in clotheslines, with denim shorts and Sunday dresses of all sizes hanging off them like ornaments.

But the houses themselves command the most attention. They look as if someone took a bunch of starter homes and glued them together with communal kitchens and shared garages. The structures’ aesthetics vary, from utilitarian ranches with unpainted decks to well-kept A-frames with manicured lawns. And virtually every structure is in a state of renovation, expanding and shape-shifting to make room for more beds, more groceries, more children — and more wives.

This is Rocky Ridge, Utah, one of the fastest-growing polygamist communes in the country, and an unlikely symbol of a genealogical subplot that links America's two main presidential candidates. While neither Mitt Romney nor Barack Obama is keen to talk about it publicly, the family trees of both are rooted in polygamy, a practice that, for each candidate, has defined generations of family history....