At a recent canvassing event for mayoral candidate Zohran Mamdani in Sunnyside, Queens, a campaign staffer kicked things off by asking how many of the three dozen volunteers on hand were about to go knock doors for him for the first time. Nearly half of the canvassers raised their hands in response. To the surprise of many in New York’s political establishment, Mamdani, 33, has leap-frogged other more-established candidates to become the leading progressive running in the June 24 primary. Polls have shown him consistently ranking as the second-place candidate behind front-runner Andrew Cuomo, and some recent surveys even indicate...