Although these nuances will baffle many voters, who can't understand why a candidate's beliefs about the Trinity or the Bible should matter at all, theological differences are key for evangelicals, according to researchers at Vanderbilt and Claremont universities. Their poll, released Wednesday, found that for conservative evangelicals surveyed, Mormons rank nearly as objectionable as atheists.

"When we say Romney is a Christian and Mormons say they are Christian," Southern evangelical respondents "become very defensive," reported Brett Benson, assistant professor of political science at Vanderbilt, who is a member of The Church of Jesus Christ of Latter-day Saints.

The Christian belief in the Trinity has been an issue in three elections — in 1800, 1908 and today — according to Forrest Church, a Unitarian minister in Manhattan and author of "So Help Me God: The Founding Fathers and the First Great Battle Over Church and State."

"The question is: 'Is Jesus God?' That's the kicker of the Trinity," Church explains. "Do you believe in the full divinity of Jesus and that only through Jesus can you be saved? If your whole faith is based on that, the moment you have a unitarian — whether you be a Universalist Unitarian, a Muslim unitarian, a Mormon unitarian — you have lowered that status of Jesus from his full divinity."

"These things become incredibly emotional," because the implication is that a nation without a leader who believes that Jesus is God will not be saved. "People argued when Jefferson was elected that Armageddon would come here and now."

If you go back to the 3rd and 4th centuries post-Christ, says the Rev. Wade Halva of Cottonwood Presbyterian, "the Trinity was the litmus test for orthodoxy." Today, he says, "it's kind of a secret handshake," a way of saying "this is what it means to be a 'real Christian."' But it's also more than just a code. Evangelicals believe that "if we don't agree about who God is, we won't agree about what God calls us to do," he says.

Whole books have been written to explain the Trinity. Here's how Franklin Evans used to explain the Trinitarian vs. unitarian view to his Methodist confirmation classes: "If God, Jesus and the Holy Spirit were coming to dinner, how many places would you set?" For people who believe in the Trinity, the answer would be "one." The Trinitarian view is that God, Jesus and the Holy Spirit are one divine being composed of three persons, explains the Rev. Pat Edwards of Bountiful's Grace Baptist Church.

Mormons, on the other hand, believe that God the Father, and his literal son, Jesus Christ, are separate beings with physical bodies of flesh and bone like those of men, only perfected. Together with the Holy Ghost, "a personage of spirit" according to LDS scripture, the three members of the Godhead are "one in purpose" but not in body.