Once Barack Obama emerged as the Democratic candidate, it was clear the presidential contest would become a referendum on race. It was not, however, supposed to be a journey into the terrain of religious fears and prejudice. But because many Americans think Obama is not what he actually is, it has become that. Beneath the candidate's Christian exterior, they suspect, beats the heart of a Muslim. Obama has tried vigorously to rebut those suspicions, but they now seem stronger than ever. Between March and July, reports the Pew Research Center, the percentage of people who believe Obama is Muslim increased from 10 to 12 percent.

Of course, even if he were a Muslim, that should be no big deal. In a country that officially separates church and state, a man's religious beliefs are his own affair. Still, nearly half a century after John F. Kennedy became America's first (and, thus far, only) Roman Catholic president, we haven't fully accepted the notion that all religions should have equal access to the Oval Office.

At the start of the political season, when Mitt Romney seemed to have a shot at the Republican nomination, pollsters sought to determine whether his Mormonism might hurt him. Nearly a third of voters, they found, were less likely to support a Mormon. But some 45 percent were wary of Muslim candidates. For Obama, that is a potential problem—particularly in a race that shows ever more signs of being extremely close.

Obama has accused his antagonists of fearmongering. Professing indignation, John McCain's campaign manager accused Obama of playing "the race card." It's not clear Obama was playing any such card—certainly not to the extent that the McCain camp was in trying to make a racial issue out of what arguably was not. Instead, Obama seemed to be expressing frustration with a real phenomenon: the attempt by enemies—not necessarily McCain himself—to turn him into something frightful. And what is more frightening, in this post-9/11 age, than the face of radical Islam?

Obama does have some Muslim roots. His father was once a practicing Muslim, then apparently became disenchanted with the religion. But the genesis of the current misconception lies less with Obama's history than with what is, in effect, a disinformation campaign. Ever since his entry into the race, a viral (and false) e-mail has been painting Obama as the product of the radical branch of Islam "that created the Muslim terrorists who are now waging Jihad on the industrialized world." For many voters, Obama's admittedly foreign background makes the claim plausible.

The campaign has been ramped up another notch. In a newly released book, author Jerome Corsi (who coauthored the anti-John Kerry "Swift Boat" attack of 2004) makes Obama out to be a one-time Muslim who likely took up Christianity for purely political reasons. Floyd Brown, who claims credit for the Willie Horton ad of 1988, has also written an anti-Obama book, linking him to "anti-Semitic people" and Nation of Islam leader Louis Farrakhan.