A similar question, asked in a Los Angeles Times/Bloomberg poll in December, put the number at 14 percent. And perhaps most relevant to Romney's chances of getting the GOP nomination, that poll found that 25 percent of white Evangelical Protestants – a key constituency in the Republican primaries – are unwilling to vote for a Mormon.
"That's not insurmountable, but it's worth paying attention to," says John Green, a senior fellow at the Pew Forum on Religion and Public Life. Some analysts may be overstating the impact of Romney's religious affiliation, he adds, "but it will be incumbent upon him to put it to rest."
The Romney campaign says he is seriously considering delivering a speech that addresses the religion question head on, the way John Kennedy addressed his Catholicism in a speech before the 1960 election. As with candidate Kennedy, Romney faces concerns that he would have dual allegiances, to both his church and the Constitution.
But for Romney, the Mormon issue has another dimension: Some Evangelicals view the religion as a cult, and even non-Christian; there is resentment over recruitment of Evangelicals into the church formally known as the Church of Jesus Christ of Latter-day Saints.
Asked about his religion Wednesday on NBC's "Today Show," Romney said, "Well, I think I've found that people across this country want a person of faith to lead the country. And they don't particularly care as much about the brand of faith as they do about the values the person has. And my values are as American as you can imagine."
Romney has spent months wooing religious conservative leaders to ease their comfort level not only with his particular faith, but also with another dimension of his campaign that may prove to be a larger hurdle among conservative GOP primary voters: Romney's wholesale change of view on top social issues – abortion, gay rights, and stem cell research.