Upstart challengers in political races often begin with the goal of drawing new or disillusioned voters to their cause. They build excitement, often on social media, but inevitably fall short.

Zohran Mamdani proved different.

He hit the ground running in the mayor’s race, talking to disaffected New Yorkers in Queens and the Bronx who voted for President Trump. He repeatedly visited mosques to hear the concerns of Muslims, hoping to provide a reason for the uninvolved to register to vote.

He won over progressives with a populist message of making the city more affordable, in part by asking corporations and the wealthy to pay more, and he spread his vision through viral social media videos. He accumulated an army of volunteers and small donors, helping his campaign knock on more than one million doors.

The polls were slow to capture his momentum, but he was building something the city had not really seen before: a winning citywide campaign for mayor, built from nothing in a matter of months.

Mr. Mamdani, the likely Democratic primary winner, still faces what is expected to be another bruising general election in November with a broader electorate. His ability to sustain his momentum will be tested, especially with business leaders already plotting how to undermine him.

He will need to further the success he found in immigrant neighborhoods like Kensington in Brooklyn, which is known as Little Bangladesh, and in the enclaves of young professionals, like Long Island City in Queens.