Zohran Mamdani’s dramatic, strong showing in New York City’s Democratic mayoral primary, where he forced a concession from former Gov. Andrew Cuomo, amounts to a massive shot in the arm for progressives and other Democrats who have been imploring their party’s elder statesmen to step aside for a new generation of leaders. Mamdani ran his campaign as an unapologetic progressive against an established favorite who argued his pragmatism would best meet the moment; a 33-year-old fresh face against a field of experienced candidates; a democratic socialist at a time when many Democrats worry whether that moniker alienates them from swing...