In February 2016, Giulianna Di Lauro, a Latino outreach strategist for Senator Bernie Sanders’s presidential operation, complained to her supervisor that she had been harassed by a campaign surrogate whom she drove to events ahead of the Democratic primary in Nevada. She said the surrogate told her she had “beautiful curly hair” and asked if he could touch it, Ms. Di Lauro said in an interview. Thinking he would just touch a strand, she consented. But she said that he ran his hand through her hair in a “sexual way” and continued to grab, touch and “push my boundaries” for...