The messages convey a sense of urgency, and are coming with increasing frequency. They are short, focused reactions to the latest “outrage” committed by President Trump. Some end by asking for money, some urge participation in protests. All read as if they are sent from the official headquarters of the resistance. Hillary Clinton is up to something. Five times in the last month alone, she sent e-mails touting her super PAC’s role in combating President Trump. Most seized on headline events, such as the family-separation issue at the southern border. Under the message line, “horrific,” she wrote June 18: “This...