William C. Thompson Jr., the sole African-American in this year’s New York City mayoral contest, rarely talks about race. In a city of flame-throwing orators, he has long favored a political path of conciliation over confrontation. But on Sunday, Mr. Thompson arrived before a mostly black congregation in a storefront church and issued a fierce denunciation of the policies of the New York Police Department, saying they are drawn from the same racial suspicions that drove George Zimmerman to hunt down Trayvon Martin. “Here in New York City, we have institutionalized Mr. Zimmerman’s suspicion with a policy that all but...