Overall, being a Mormon is hardly an asset for presidential candidates, but it is not a deal-breaker for most Americans. A quarter of Americans say they would be less likely to support a presidential candidate who is Mormon, while 68 percent say it would not make a difference. For perspective, about the same number say they would be less likely to vote for a candidate who has used marijuana in the past.

But an important group within the Republican base, white evangelical Protestants, is more uncomfortable with the idea of a Mormon candidate than are other Republicans. Among all Republicans and Republican-leaning independents, 31 percent of white evangelicals say they would be less likely to vote for a Mormon; that compares with 15 percent of other Republicans, according to a May survey. This gap is as large as it was four years ago.

At that time, our polling found that white evangelical Protestants were more likely than non-evangelical white Protestants to view the Mormon religion as very different from their own. And just 40 percent of all white evangelicals viewed Mormons as Christians; far more non-evangelical white Protestants and Catholics said that Mormons were Christians.

These findings have potentially troubling implications for Jon Huntsman and Mitt Romney, particularly when one considers the G.O.P. primary landscape. Exit polls from 2008 show that 60 percent of those who voted in the G.O.P. Iowa caucuses, and an identical percentage of Republican voters in the South Carolina primary, were evangelicals.

The Muslim and Mormon religions have gained increasing national visibility in recent years. Yet most Americans say they know little or nothing about either religion’s practices, and large majorities say that their own religion is very different from Islam and the Mormon religion.

A new national survey reveals some notable similarities, as well as major differences, in the ways that Americans view these faiths and their followers. Public impressions of both religions are hazy — 58% say they know little or nothing about Islam’s practices, while 51% have little or no awareness of the precepts and practices of Mormonism. The number of people who say they know little or nothing about Islam has changed very little since 2001.

Most Americans believe that their own religion has little in common with either Islam or the Church of Jesus Christ of Latter-day Saints. Fully 70% say that their religion is very different from Islam, while 62% say this about the Mormon religion. The proportion who say that Islam has little or nothing in common with their own religion has increased substantially since 2005 (from 59% to 70%).

The latest national survey by the Pew Research Center for the People & the Press and the Pew Forum on Religion & Public Life, conducted Aug. 1-18 among 3,002 adults, finds that overall evaluations of Mormons and Muslim Americans are on balance positive: 53% say they have a favorable opinion of Mormons, while an identical percentage views Muslim Americans favorably. As in past surveys, more people have a positive impression of “Muslim Americans” (53%) than of “Muslims” (43%).

Despite these similarities, there also are clear differences in public attitudes about Islam and Mormonism. These are reflected in the single-word descriptions people use in summarizing their impressions of each religion. Twice as many people use negative words as positive words to describe their impressions of the Muslim religion (30% vs. 15%). The most frequently used negative word to describe Islam is “fanatic,” with “radical” and “terror” often mentioned as well. Among the positive terms, “devout” or some variant is the most frequently cited.

The words that people use to describe the Mormon religion are, on balance, more positive. Nearly a quarter (23%) gives a positive word to describe their impression of the Mormon religion while 27% use a negative term. Although the Church of Jesus Christ of Latter-day Saints banned polygamy almost a century ago, many Americans still associate the church with this practice. The most commonly used negative words to describe Mormonism are “polygamy,” “bigamy” or some other reference to plural marriage. Among positive words used to describe the Mormon religion, “family” — or some variant of the term — is the most frequent response.

Public views of other religious groups have changed little over the past few years. About three-quarters of those polled have a favorable opinion of Jews and Catholics (76% each), while substantially fewer are favorable toward evangelical Christians (60%). Atheists are viewed far more negatively, with just 35% holding a positive view and 53% saying they have an unfavorable opinion.

The survey also finds that, two years after Pope Benedict XVI was installed as spiritual leader of the world’s Catholics, the pontiff is viewed favorably by nearly three-quarters (73%) of those familiar enough to offer an opinion. However, significantly fewer people say they have a favorable opinion of the pontiff than expressed positive opinions of Pope Benedict’s predecessor, Pope John Paul II, during his more than two decades as pope (86% in 1996).

Moreover, nearly half (46%) of those who have heard at least a little about Pope Benedict XVI say he is doing only a fair or poor job at promoting good relations with other major religions; just 38% say the pope is doing an excellent or good job in this regard. Catholics themselves are divided ideologically over the pope’s performance in fostering ties with other religions: 63% of self-identified conservative Catholics say the pope has done well in promoting good interfaith relations, but just 50% of moderate Catholics and 45% of liberal Catholics agree.

People who have heard at least a little about Pope Benedict are in general agreement about the pope’s own ideological leanings: 56% say he is either very conservative (20%) or conservative (36%); 17% say the pope is a moderate, while just 5% view him as a liberal. And among Catholics, fully 68% say Pope Benedict is a conservative.