Zohran Mamdani won the Democratic primary for mayor of New York City by focusing on affordability. But he also took aim at wealthy New Yorkers who he argued were not paying their fair share of taxes.

On Saturday night, Mr. Mamdani will officially join the national Fighting Oligarchy tour, appearing at a town hall in Brooklyn alongside Senator Bernie Sanders of Vermont, one of his political heroes.

The event is meant to excite Mr. Mamdani’s base of progressive voters as he fends off attacks heading into the general election in November. It also shows the durability of the movement led by Mr. Sanders, who turns 84 on Monday and has sought to elevate younger leaders like Mr. Mamdani, 33, and Representative Alexandria Ocasio-Cortez, 35, during his barnstorm across the United States.

The town hall was billed by Mr. Mamdani, a state assemblyman and democratic socialist, and Mr. Sanders as a moment to rally against President Trump’s second term agenda.

“When we stand together,” Mr. Sanders said, “we can defeat authoritarianism and create an economy that works for all our people, not just the privileged few.”

The event comes as Mr. Mamdani faces a series of challenges during the final stretch of the race: a push by Mr. Trump and others to narrow the field to stop Mr. Mamdani, resistance from prominent Democrats over endorsing him and pressure on Mr. Mamdani to distance himself from the Democratic Socialists of America to appeal to a broader audience.