Where race is concerned, I sometimes think of the president as the Peter Sellers character in “Dr. Strangelove.” Sellers plays a closet Nazi whose left arm—quite involuntarily—keeps springing up into the Heil Hitler salute. We see him in his wheelchair, his right arm—the good and decent arm—struggling to keep the Nazi arm down so that no one will know the truth of his inner life. These wrestling matches between the good and bad arms were hysterically funny.

When I saw Mr. Obama—with every escape route available to him—wade right into the Gates affair at the end of his health-care news conference, I knew that his demon arm had momentarily won out over his good arm. It broke completely free—into full salute—in the “acted stupidly” comment that he made in reference to the Cambridge police’s handling of the matter. Here was the implication that whites were such clumsy and incorrigible racists that even the most highly achieved blacks lived in constant peril of racial humiliation. This was a cultural narrative, a politics, and in the end it was a bigotry. It let white Americans see a president who doubted them.

Mr. Obama’s “post-racialism” was a promise to operate outside of tired cultural narratives. But he has a demon arm of reflexive racialism—identity politics, Rev. Jeremiah Wright, and now Skip Gates.