Of the 44 U.S. presidents, all but a handful have been affiliated with a relatively narrow list of traditional Protestant denominations.
Eleven were Episcopalians (12 if you count Thomas Jefferson, whose adult beliefs are a subject of debate), eight were Presbyterians, four were Methodists and four were Baptists. Others included Congregationalists, Dutch Reformed and Disciples of Christ.
President Obama attended Trinity United Church of Christ in Chicago, a congregation with traditional Protestant roots despite its untraditional pastor, the Rev. Jeremiah A. Wright Jr. In Washington, Obama has attended services at mostly black Protestant churches.
The only chief executive whose roots were clearly outside that mainstream tradition was John F. Kennedy, a Roman Catholic.
But among the leading candidates for this year's Republican presidential nomination, not one is a member of the Protestant denominations that for so long have dominated American political culture.
Two of the potential candidates are Mormons (former Massachusetts Gov. Mitt Romney and former Utah Gov. Jon Huntsman Jr.); one is a member of an interdenominational evangelical church (former Minnesota Gov. Tim Pawlenty); two others are Catholics (former House Speaker Newt Gingrich and former Sen. Rick Santorum). Rep. Michele Bachmann, who says she's considering the race, worships at an evangelical Lutheran church; if elected, she'd be the first Lutheran president.
But no matter who wins from this list, it won't be an Episcopalian, a Presbyterian or a Methodist.
The denominational diversity of the GOP field reflects a trend that has been building for half a century: the decline of the "mainline" churches' size and influence. Among Protestants, evangelical congregations have taken off, and the old mainline denominations have been shrinking.
And the American establishment is no longer predictably Protestant. To take a striking example, the Supreme Court, once a redoubt of Episcopalians and Presbyterians, now counts six Catholics and three Jews but no Protestant on its bench.
Once a church of immigrants, Catholicism has been fully absorbed into the American elite. Catholic politicians, who used to be almost exclusively Democrats, are now among the leaders of both parties. Today, the Republican House speaker is Catholic (and the Republican House majority leader is Jewish).
The greater diversity among politicians also reflects more tolerance on the part of American voters, at least toward some sects. In 1960, candidate Kennedy had to respond to Protestant fears that a Catholic president would take direction from the Vatican; today, conservative Catholic candidates count on winning evangelical Protestant votes.
These shifting religious sands have brought about a structural change in the relationship between politics and religion in the United States. Once, the key religious divides were among Protestants, Catholics and Jews; now they're between conservatives and liberals within every faith. On election day, conservative Protestants have more in common with conservative Catholics than with liberal Presbyterians.
There's still a "God gap" in American politics between the religiously observant (who tend to vote Republican) and the less observant (who tend to vote Democratic), but it now crosses denominational lines. "If I know whether you say grace before meals every day, I can probably predict how you vote," said Notre Dame political scientist David E. Campbell. (About 44% of Americans say grace, and most of them vote Republican.)
That also helps explain why so many Republican candidates come from conservative religious backgrounds — whether Mormon, Catholic or evangelical — instead of the more liberal traditional GOP denominations, which now stand outside the party's conservative mainstream.
But there's one big, intriguing exception to this picture: the Church of Jesus Christ of Latter-day Saints, popularly known as Mormons.
"Mormon candidates like Romney still face a stained-glass ceiling," said Campbell, himself a Mormon. "It's an obstacle, it's a big one, and it didn't go away in 2008."
In public opinion polls, between one-quarter and one-third of voters say they'd be reluctant to support a Mormon for president. That's more than say they have qualms about a Jewish candidate. (In the same polls, Muslims fare even worse than Mormons, but not much worse.)
Republican voters — the ones Romney and Huntsman will face in the GOP primaries — are more likely than Democrats or independents to say they wouldn't vote for a Mormon. Among evangelicals, more than half say they don't like the idea of a Mormon president.
The underlying problem, Campbell says, is religious, not political. Most evangelicals don't believe Mormons are Christians. (Mormons believe in the divinity of Jesus, but their theology differs from evangelical Christianity in several ways, beginning with their belief in Mormon scriptures in addition to the New Testament.) For many of those voters, a candidate they perceive as non-Christian appears to be a step too far.
Likewise, most voters say they want their candidates to profess some kind of religious faith. The United States is the most religious of any major industrialized country; a larger percentage of Americans attend a weekly religious service than Iranians — and Iran is a theocracy. As a result, American politics is one line of work where avowed atheists need not apply.
"You can tell when an election is coming: when all the candidates are going to church again," said Michael Cromartie of the Ethics and Public Policy Center, a Washington think tank.
American politics are more tolerant of different religious traditions than half a century ago, but that tolerance still has limits.
The door has opened for Catholics and even Jews to run for president, but Mormons still face a high hurdle — not to mention Muslims. And atheists? Don't even ask.
So, yes, there's a de facto religious test for office. It's not administered by the government; it's a matter of voters' comfort level.
But if the last half-century is any guide, the test will grow easier over time. The trend is toward ever greater openness. The arc of American history is long, but it bends toward tolerance.