In a lifetime of activism, I have canvassed and phone-banked, raised money, and twisted arms for dozens of political candidates. Zohran Mamdani, the 33-year-old Indian-Ugandan democratic socialist and presumptive winner of the New York City Democratic mayoral primary, is the only one I’ve both supported without reservation and believed could win. Volunteering for a campaign always teaches you something. Often, it’s discouraging – like the moment my partner and I saw Hillary Clinton’s team selling lawn signs for $25 instead of blanketing Philadelphia by distributing them free, and predicted she’d lose. But the lessons of Zohran’s victory are hopeful for...