By contrast, in 1960, when John F. Kennedy was elected, Catholics constituted a voting bloc 10 times larger than Mormons are today. Then, most Catholics were Democrats — it’s about evenly divided now — and there still were enough Republican-leaning Catholics who voted for Mr. Kennedy to offset the resistance of some Protestants.

Moreover, it isn’t just evangelicals who harbor reservations. In a national poll by Bloomberg News this month, a plurality of voters had an unfavorable view of the Mormon Church; they held positive attitudes about the Catholic Church by an almost 2-to-1 ratio. The negativity about Mormons was expressed by a plurality of important swing voters, political independents and married women with children.

There’s also a question, the anti-Obama effect notwithstanding, of whether born-again Christians would enthusiastically rally behind Mr. Romney if he’s the nominee. Their reservations are clear. In a number of primaries, in the Midwest as well as the South, Mr. Santorum beat Mr. Romney by double digits among evangelicals, according to exit polls.

More striking was the behavior of those who say a candidate’s religious beliefs matter a great deal. Mr. Romney, a man of demonstrable religious involvement, should appeal to these voters. Instead, he was trounced by Mr. Santorum among those who say religion matters a great deal: 52 percent to 21 percent in Ohio; 51 percent to 17 percent in Tennessee; and 47 percent to 16 percent in Alabama. In the primary last week in Illinois, which Mr. Romney carried easily, he was pummeled again among these voters.

The most plausible explanation is anti-Mormonism, which may transcend doctrinal and cultural attitudes. Southern Baptists and Mormons are competing for similar converts: “There is a high cost associated with mainstreaming your competition,” says Brett Benson, a political science professor at Vanderbilt who works with Dr. Geer on these issues.

There are three strains of political anti-Mormonism, says Dr. Benson, whose great-uncle, the late Ezra Taft Benson, was the head of the Mormon church: evangelicals; political left-wingers who dislike the conservatism of members; and a middle group, not that well-informed, that considers the religion “weird or strange.”

On the campaign trail, Mr. Romney occasionally talks about faith and has mentioned his experiences in France in the late 1960s when he was a missionary; he never utters the word Mormon.