For Sens. Kamala D. Harris and Cory A. Booker, Democratic presidential candidates who were promoting an anti-lynching bill, the “attack” on actor Jussie Smollett in Chicago looked like a horrible yet opportune talking point. Instead, the apparent hoax is turning out to be a warning to 2020 Democratic candidates about singling out people for martyrdom, including the woman who accused Supreme Court Justice Brett M. Kavanaugh of sexual assault and the drum-banging American Indian who confronted Catholic high school students on the steps of the Lincoln Memorial. In the rush to speak too soon about the Smollett case, Ms. Harris...