In looking back at that history, you wish that a candidate had responded not only with, “No, I don’t have any black ancestor,” but also with, “So what if I did?”

Likewise, with countless people today spreading scurrilous rumors that Mr. Obama is a Muslim, the most appropriate response is a denial followed by: And so what if he were?

Granted, that’s not politically realistic as a comeback. A 2007 Gallup poll found that 94 percent of Americans said they would vote for a black candidate for president and 88 percent for a woman. In contrast, a Los Angeles Times poll in 2006 found that only 34 percent of respondents said they could vote for a Muslim for president.

Even if a prejudice is directed to a matter of choice, like religion or long hair, it’s still prejudice. It’s possible to believe that Catholics have every right to be president while opposing a particular Catholic candidate who would ban contraception; likewise, it’s possible to believe that Muslims have every right to hold office without necessarily embracing the candidacy of particular Muslims who advocate enveloping all women in burkas.

To his credit, Mr. Obama has spoken respectfully of Islam (he told me last year, on the record, that the Muslim call to prayer is “one of the prettiest sounds on earth at sunset”). If he were to go further  “and so what if I were Muslim?”  many Americans would see that as confirmation that he is a Sunni terrorist agent of Al Qaeda who is part of a 9/11 backup plan: If you can’t reach the White House with a hijacked plane, then storm the Oval Office through the ballot box.

This is a case where Hillary Rodham Clinton and John McCain should take the initiative and denounce the fear-mongering about Mr. Obama as hate speech. The wink-wink references to “Barack Hussein Obama” and lies about his going to a madrassa are the religious equivalent of racial slurs, and Mr. McCain and Mrs. Clinton should denounce them in the strongest terms. This is their chance to show leadership.

When Mrs. Clinton was asked in a television interview a week ago whether Mr. Obama is a Muslim, she denied it firmly  but then added, most unfortunately, “as far as I know.” To his credit, Mr. McCain scolded a radio host who repeatedly referred to “Barack Hussein Obama” and later called him a Manchurian candidate.

Martin Luther wasn’t a model of tolerance but even he took the position that, “I’d rather be ruled by a wise Turk than a foolish Christian.” In this presidential campaign, we should at least aspire to be as open-minded as 16th-century Germans.