He has proposed an ambitious program of benefits intended to alleviate New York City’s affordability crisis, almost none of which he can unilaterally enact or pay for. He faces significant financial risks as President Donald Trump’s administration threatens to slash billions of dollars in federal funding for city and state social safety net programs. And his charisma, youth and biography — he will be New York’s youngest mayor in a century and the first Muslim and person of South Asian descent to hold the office — mean millions are watching his every move. Mamdani’s election victory polarized the Democratic Party,...