There are major doctrinal differences between the Mormons and Christian churches, which are essentially related to Joseph Smith and the Book of Mormon.
Long story short -- Smith was an early-19th-century charismatic leader who made up the Book of Mormon, convinced people to believe in its fantastic provenance, Meso-American fables, and the concept of Smith as a modern prophet, and thereby created the Mormon church.
From the get-go in the 1830s, Smith sent missionaries bearing the Mormon gospel all throughout America and to Europe.
Then as now, Mormon missionaries target Christians for conversion to their semi-Christian church.
Most Christian churches do not like their members being poached by Mormon missionaries.
Especially the largest Protestant Christian denomination in the USA -- the Southern Baptists.
First, it's pretty obvious that Mormon missionaries are not targeting American atheists, agnostics, Jews, Muslims, etc., for conversion.
That's a bridge too far -- much easier is convincing Christians that Joseph Smith's new-and-improved version of Christianity is the "only true and living church."
In the early 1980s, Southern Baptist Convention leaders discovered — much to their horror — that 40 percent of Mormonism's 217,000 converts in 1980 came from Baptist backgrounds. More than 150 Mormon missionaries had descended on the northern Georgia area alone, a Southern Baptist magazine noted warily in 1982, and they found Southern Baptists among their most promising targets.Here's just one example of Mormon missionaries "lying for the Lord" in their campaign to convert Christians:
A Mormon missionary admitted by letter that he was recruiting for the Mormon church at a Christian singles group.IMHO, Southern Baptists dislike Mormons in part because they have been competitors for tithing true-believers for a long time.
When the pastor confronted him, he outright lied to the pastor until the pastor showed him his own letter. Caught in embarrassment, he then apologized, only to
turn around and repeat the same thing at a later time in the same church.
Southern Baptists may quibble about Mormon doctrine and practices, but it's really all about the money, as it is for almost every religion, new and old.
Both Mormons and Southern Baptists are, generally, very conservative white Republicans, and will vote for the GOP presidential candidate in November.
But one major reason Romney is suffering in the primaries is because many in the GOP's Southern Baptist/Religious Right/tea party base have a problem with any Mormon presidential candidate that goes beyond Book of Mormon fables, Masonic-based temple ceremonies, and baptizing the dead against their families' wishes.
Though Southern Baptists are themselves evangelical proselytizers, they resent Mormons who actively proselytize their members.