Zohran Mamdani declared victory in New York City’s Democratic mayoral primary Tuesday night after Andrew Cuomo conceded the race in a stunning upset, as the young, progressive upstart who was virtually unknown when the contest began built a substantial lead over the more experienced but scandal-scarred former governor.

Zohran Mamdani, a state assemblyman and democratic socialist, had a commanding lead over former Gov. Andrew M. Cuomo, who conceded in the Democratic primary.