Republican Sen. John McCain, an Episcopalian, says, "I do believe that we are unique and that God loves us." Former Arkansas Gov. Mike Huckabee, an ordained Baptist minister, emphasizes his belief that "God created the heavens and the earth. To me, it's pretty simple."

Unlike the others, former New York Mayor Rudolph Giuliani, a divorced Roman Catholic who favors abortion rights, sidesteps such questions, claiming one's relationship with God is a private matter. But he attended Catholic schools and at one point considered being a priest.

Clearly, the religious issue is the most problematic for Romney. Polls suggest he faces continued misgivings over his faith. An ABC News-Washington Post poll conducted July 18-21 showed that 32 percent of those who said they leaned Republican described themselves as "uncomfortable" with the idea of a Mormon president.

An earlier poll by the Pew Research Center said 30 percent of respondents said they would be less likely to vote for a candidate who was Mormon. The negative sentiment rose to 46 percent for Muslim candidates and to 63 percent for a candidate who "doesn't believe in God."

Pollster Andrew Kohut, Pew's director, said that between the late 1960s, when Romney's father ran, and now there has been "one of the great transformations of our era. There is more mixing of religion and politics than there was then. As a consequence, people scrutinize Mormonism — or any other religion — more closely than back then."

He cites the growing influence of the Christian right, the political activism of tele-evangelists and a trend that has seen a steady migration of Christian conservatives into the GOP fold, particularly in the South.

"When the South changed, it brought the evangelicals with it," Kohut said.

The links between religion and governance intensified with the presidency of George W. Bush, said Joan Konner, former dean of the Columbia Journalism School. "He brought it up when he ran for office, and he said his favorite philosopher, in answer to a question in a debate, was Jesus.

"And then he followed up on that by faith-based public funding and various other actions that started to erode what Americans took for granted as the separation between church and state," said Konner, who has studied the interaction between religion and politics and is the author of "The Atheist's Bible."

George W. Romney was a politically moderate former governor of Michigan and auto-industry executive when he sought the 1968 GOP presidential nomination. Scant mention was made of his Mormonism in news accounts at the time and it appeared to be a non-issue in the race.