In 2005, one year into her job as San Francisco district attorney, Kamala Harris was showing a new hire around the office. Ms. Harris had recruited Lateefah Simon, a 28-year-old racial justice activist, to lead a new program aimed at keeping first-time drug offenders out of jail. As the two women walked the halls, they stopped in front of a wall lined with photographs of Ms. Harris’s predecessors — all of them white men. “The expectation of our community is that I’m going to fix all the havoc,” Ms. Harris said, according to Ms. Simon’s recollection. “They’re going to want...