After a deluge of emails from readers insisting that Mormons are indeed Christians, all I can say is that they are Christians of a very different stripe than most others. The strains between evangelical Christians and Mormons are real, have always existed, and will empirically play a part in restraining some evangelical Christians from supporting Romney.
The bigger problem for Mormons in public office - especially national public office - seems to me to be the long period of racial discrimination in the Mormon Church. While the founder, Joseph Smith, was an abolitionist (after some early prevarication), his sect subsequently banned all African-Americans from the Mormon priesthood. (For Wikipedia's discussion of this history, click here.) Brigham Young, whose eponymous university Romney attended, was particularly emphatic about God's damnation of Africans and African-Americans:
Almost immediately after the death of Joseph Smith, Brigham Young returned to the old rhetoric concerning African Americans and the curse of Ham. Brigham Young actually subscribed to another common, although less popular theory that the descendants of Ham were also the descendants of Cain, Ham having married a woman of that race ... Some scholars interpreted the mark placed upon Cain as the black skin of the African peoples.
President Young remained very strict in his interpretation. He believed the curse included not only priesthood restriction but also black skin and perpetual servitude. He believed the curse could be removed only by God and that the Civil War effort to free the slaves was in vain. He believed that the Civil War would destroy the United States and spread to every nation until the Saints could return to Missouri and build a temple in Jackson County. The slaves could be freed only by a decree from God by revelation to the prophet accompanied by the removal of the mark of Cain. It was not expected before the millennium.
The first statement linking priesthood denial with the curse of Cain is dated February 13, 1849. It was given by Brigham Young in response to the question, "What chance is there for the redemption of the Negro?" Young responded, "The Lord had cursed Cain's seed with blackness and prohibited them the Priesthood."
President Young never cited Joseph Smith for the source of his doctrine but stated it in his own authority as a prophet, even in the name of Jesus Christ on a least one occasion, as did multiple apostles, including Parley P. Pratt and Heber C. Kimball. In 1852, while addressing the state legislature, Young stated: "Any man having one drop of the seed of [Cain]...in him cannot hold the priesthood and if no other Prophet ever spoke it before I will say it now in the name of Jesus Christ I know it is true and others know it."
They only changed this position as late as 1978! Romney was part of a church that barred blacks from the priesthood for his first 31 years. The church policy was amended when expansion into Brazil made racial classifications for the priesthood almost impossible because of such high levels of miscegenation.
Of course, other Christian churches also exclude groups of people from the priesthood. Catholics still bar women, and, since last year, also celibate gay men. But the racial exclusion for such a long period of time is unique to Mormonism, so far as I know, and the racial question in America is still extremely potent. The only sect I can think of as equivalent is the Nation of Islam - in reverse. I don't know if Romney has addressed the question of Mormon racism in its historical practices, or whether he has a record of opposing it in his twenties, when he was a missionary for a racist church. But it strikes me as a matter that will require addressing. It sure won't help increase African-American votes for the GOP.