Tired of questions about his religion, Mitt Romney told off a pushy talk-radio host in Iowa last summer: "I'm not running to talk about Mormonism."

Like that testy exchange itself, captured on YouTube, the questions have not gone away, and today the Republican presidential candidate plans a much-anticipated speech on the role of religion in public life.

It's a chance for Romney, a former governor of Massachusetts, to clear the air on an issue that suddenly seems to be threatening his candidacy in Iowa, which holds the first caucus of the presidential race Jan. 3.

The moment calls to mind John F. Kennedy's 1960 address aimed at convincing Protestants that he would not take orders from the pope if he were elected the first Catholic president.

Romney's political problem is considered different, stemming from an acute distrust of Mormonism among many evangelical Christians, about a third of voters in Republican nominating contests. Some even consider the Mormon faith a cult.

The Church of Jesus Christ of Latter-day Saints, the faith's formal name, has distinctive tenets that some Christians consider heretical - including its own set of scriptures to supplement the Bible - and a controversial history that includes polygamy.

Repeated polls during the last 10 months have found a large number of Americans who say they would be less likely to vote for a presidential candidate who is Mormon. A recent Pew survey pegged the number at 25 percent.

Forty-one percent of evangelicals who attend church at least once a week said in the Pew poll that they would be less likely to vote for a Mormon.

"A vote for Romney is a vote for Satan," Bill Keller, a Florida evangelist, wrote in a widely circulated e-mail this year. In a statement yesterday, he said he distrusts Mormons because they claim to be Christian but believe the Bible is incomplete, adding that he does not want souls to be lost to their "deception."

Romney had been leading the GOP pack in Iowa for months, but several recent surveys have found him tied with Mike Huckabee, a former Arkansas governor and Baptist pastor. A Des Moines Register poll Sunday showed Huckabee leading Romney, 38 percent to 22 percent, among respondents who considered themselves born-again Christians, a turnaround from its October survey.

"He might not have had to do this speech if Huckabee's candidacy had not provided the evangelicals in Iowa a place to go and not feel they are bigoted," said Jan Shipps, a leading historian of Mormonism from Indiana.

Campaigning yesterday, Huckabee declined to say what he thought of Mormonism. Last week, his campaign began running a TV ad in Iowa describing him as a "Christian leader."

Romney is not the first Mormon to run for president - or the first Romney. His father, George, who was then governor of Michigan, ran unsuccessfully for the GOP nomination in 1968; the elder Romney's faith was not an issue. Former Rep. Morris Udall, an Arizona Democrat, was born Mormon but was not an active church member when he lost the 1976 Democratic nomination.

The religion's patriarch, Joseph Smith, began a third-party campaign for president in 1844, not long before he was killed by a mob.

Smith organized the church after digging up mysterious golden plates in Upstate New York and translating them into the Book of Mormon, a scripture telling how Jesus revealed himself right after his crucifixion to a lost tribe of Israel wandering in America.

Mormons believe that God was once a man, that good Mormons can become gods themselves through "eternal progression" after death, and that Christ will reign for 1,000 years from Missouri, site of the Garden of Eden, when he returns to earth.

Shortly after Smith's death, the Mormons migrated west to escape persecution and settled in Utah. The church outlawed polygamy in 1890.

A sixth-generation "DNA Mormon," Romney has emphasized beliefs that his religion shares with other Christians. He condemned polygamy in a 60 Minutes interview and calls Jesus his "personal savior," a phrase common in evangelical circles but not so among Mormons.

"The truth is, at the core of all religions is 'weird' stuff that's hard to believe," said the historian Shipps, a Methodist. Since many Christians consider Communion the body and blood of Christ, she said, "Is somebody going to go ask Mike Huckabee, 'Are you guys cannibals?' "

Romney told reporters this week that he was not planning to explain details of Mormon theology in today's address but would discuss his faith in the context of tolerance.

Kennedy struck a similar note in his 1960 speech.

Political analysts say Romney's situation is different. For one thing, Catholics were about 28 percent of the U.S. population in 1960, while Mormons, numbering about six million, are 2 percent of the population today. (There are about 14,000 Mormons in the Philadelphia region, church officials say.)

Since then, evangelicals have risen in power, particularly in the GOP, and it has become more common for voters to want to know about candidates' religious beliefs.

"Running for president is a full-time reality show, and people expect all you've got," said Kirk Jowers, director of the Hinckley Institute of Politics at the University of Utah.

Richard Land, a Southern Baptist leader, said it would be foolhardy for Romney to try to persuade evangelicals that his religion is the same as theirs.

"It's a fight he can't win with a lot of the voters he wants," Land said. "But it's not a fight he has to have."

He said he advised Romney to say that people should judge him on his record.

"I do not believe that Mormonism is an orthodox, trinitarian, apostolic Christian religion, but that would not keep me from voting for a Mormon," Land said. "I would vote for a Mormon who agreed with me on important issues like abortion and same-sex marriage over a Baptist who didn't, like Jimmy Carter."

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.