The last time religion was a factor in a presidential election was when John F. Kennedy, a Catholic, ran in 1960. Kennedy captured the presidency after defusing the issue in a Sept. 12, 1960, speech before the Greater Houston Ministerial Association, a Baptist organization.

"I believe in a president whose religious views are his own private affair, neither imposed by him upon the nation or imposed by the nation upon him as a condition to holding that office," Kennedy said in the speech.

Romney said in March that he expects his religion to be an issue if he pursues the presidency. "There ultimately will be a time when someone will go overboard, where someone will say something beyond the mark," he said. "And hopefully I will be able to rise to the occasion in a way that's memorable."

Among some voters, social concerns may be partly driving the anti-Mormon numbers, said John Green, a political scientist at the University of Akron in Ohio who studies the impact of religion on politics. "It looks like while there may be a religious factor here, it's also an ideological factor," he said. "Liberals are concerned about Mormons."

Mormons, including Romney, have supported a ban on gay marriage and limits on abortion rights and stem-cell research.

Among political groups, the highest opposition to a Mormon candidacy comes from people who describe themselves as liberal Democrats, 50 percent of whom say they wouldn't vote for a Mormon. Thirty-three percent of moderate Republicans say they wouldn't, as do 35 percent of conservative Republicans.

Minorities are more opposed to a Mormon presidential candidate than whites, with 51 percent saying they wouldn't vote for one, versus 31 percent of whites. Sixty percent of nonwhite Protestants say no to a Mormon president.

A Mormon candidacy would also likely draw some opposition from evangelical Christians, Green said. "Some evangelical churches actually label the Latter-Day Saints as a cult," he said.

Some of the church's teachings differ from those of other Christian denominations. Mormonism says that the early Christian church fell from the truth and that "in the latter days" Christ has been restoring it through modern-day prophets, starting in 1820 with Mormon church founder Joseph Smith.