Correction: An earlier version of this article incorrectly stated that the United States outlawed polygamy in 1890. That date was when the Mormon church publicly broke with polygamy; the following version has been corrected.

David Maraniss, an associate editor of The Post, is the author of “First in His Class: A Biography of Bill Clinton” and the forthcoming “Barack Obama: The Story.” This is the third column in an occasional series on the 2012 presidential candidates’ political lives.

For a biographer, the emerging matchup in the presidential campaign between Barack Hussein Obama and Willard Mitt Romney is about as American as it can get. Although the candidates are noted for their innate caution, their family histories evoke a kind of exceptionalism that defines the United States — not in some gauzy and false way, but in the reality of a national fabric woven from exotic threads. Where to start? How about polygamy?

Perhaps it is true of most people if you go back far enough, but with Obama and Romney, it can be said with certitude that neither would exist had their ancestors not lived with many wives at once.

The president’s most virulent critics have tried for years to portray him as a stranger in our midst, someone outside the comfortable mainstream of American life; a Muslim socialist born elsewhere, probably in Kenya. The mythology is wrong on all three particulars about Obama, a Christian liberal born in Hawaii, and its distortions are antithetical to historical inquiry, a manipulation of facts for ideological purposes. Yet the real story is colorful enough, and odd in a way that is foreign while familiar. Sit down long enough at any American family’s table, and some strange history is likely to be served.

The line of polygamists in Obama’s family can be traced back generations in western Kenya, where it was an accepted practice within the Luo (pronounced LOO-oh) tribe. His great-grandfather, Obama Opiyo, had five wives, including two who were sisters. His grandfather, Hussein Onyango, had at least four wives, one of whom, Akumu, gave birth to the president’s father, Barack Obama, before fleeing her abusive husband. Obama Sr. was already married when he left Kenya to study at the University of Hawaii, where he married again. His American wife-to-be, Stanley Ann Dunham, was not yet 18 and unaware of his marital situation when she became pregnant with his namesake son in 1961.

The line of polygamists in the Romney family traces back generations, when it was an accepted practice in the Church of Jesus Christ of Latter-day Saints. His paternal great-grandfathers, Miles Park Romney and Helaman Pratt, were born in the United States but lived for decades in Mexico. Pratt was a Mormon missionary there; Miles Park Romney left Utah for Mexico with a tribe of polygamous Mormons after the Mormon church broke with the practice of polygamy in 1890.

Pratt had five wives. Miles Park Romney had four, and 30 children, one of whom was Gaskell Romney. The polygamy stopped at Gaskell, who had a single wife and seven children. One of the children, George, was born in a Mormon colony in the state of Chihuahua, Mexico, although he was nonetheless a U.S. citizen. He was Mitt’s father.

For both presidential candidates to have fathers born outside the United States — what could be more American than that? And the lives of both were shaped by evangelizing branches of Christianity — one family doing the proselytizing, the other being proselytized, sort of — again, how American.

All of the Romney men were Mormon missionaries at some point; it comes with the religious territory. Old man Pratt did his missionary work in Mexico. George Romney, before becoming governor of Michigan and a candidate for president himself, took his Mormon outreach assignments in Glasgow, Edinburgh and London. His son Mitt went off in search of potential converts in France. It turned out that neither George nor Mitt was particularly good at making converts during their overseas forays. They proved to be much better at making money back home, and were happier doing so.

As a member of the Luo tribe, Hussein Onyango encountered missionaries early in his life; it came with the cultural geography of western Kenya. This was not Mormon meets African, the central conceit of the popular Broadway musical “The Book of Mormon,” which takes place in Uganda. The Latter-day Saints arrived in western Kenya long after the Seventh-Day Adventists.

It is an understatement to say that Mormons came late to a lot of things involving people with black skin. The first evangelist to reach the Obama homestead near Kendu Bay was Arthur Asa Carscallen, an Adventist pastor who grew up in Canada and ventured to western Kenya in 1906, tooling around in an elegant suit and necktie as he peddled the gospel. Carscallen learned the Luo language and built a missionary primary school atop a hill in Gendia, where he taught Luo boys, the first generation to become Westernized. Onyango was one of his students, setting the family on its unlikely path to the White House.

Years after learning English and adapting Western ways, Onyango converted to Islam. He added the name Hussein and lived out his days as a Muslim, although he did not follow all of the religion’s strictures. This is where the myth of his grandson being a Muslim, which has no factual basis, took hold. Hussein Onyango’s son, Barack Obama Sr., was an atheist by the time he reached adulthood. When I die, I will die thoroughly, he would say. Stanley Ann Dunham, the president’s mother, was not religious in a church sense but was spiritual and had a deep interest in many of the world’s religions.

Mitt Romney was born into a home where religion offered meaning and comfort. It suffused him from the beginning, requiring no searching. Barack Obama had to find that meaning and comfort. His father was nowhere in his life, and his mother’s sensibility, although admirable, left him unsatisfied and worried that he would end up like her, “free in a way that my mother was free, yet also alone in the same ways she was ultimately alone.” He had to be his own missionary to himself. The path of his life traced an arc toward home, which he found in Chicago — a community, a black church and a “traditional” American family, one wife and two kids.