Earlier this week, political leaders from the Bronx and beyond stood beside Assemblyman Zohran Mamdani in a show of support of his candidacy for mayor of New York City.

There was a notable absence. Missing from the rostrum was the county’s most powerful elected official and Mr. Mamdani’s legislative boss: the Assembly speaker, Carl Heastie, a fellow Democrat.

Mr. Heastie’s hesitance to publicly embrace the Democratic nominee reflects both the frustration some in his delegation have about Mr. Mamdani and the complex political dynamics within an ideologically diverse conference, according to conversations with more than a dozen legislators, lobbyists and other people close to the speaker.

Some lawmakers have expressed concern to Mr. Heastie about Mr. Mamdani’s membership in the Democratic Socialists of America, and how that might be used against Democratic lawmakers in swing districts. Others have singled out Mr. Mamdani’s outspoken criticism of Israel as a nonstarter.

Charles Lavine, a Nassau Democrat who is chairman of the Assembly Judiciary Committee and close to Mr. Heastie, said the speaker was sensitive to how his political decisions affect members in swing districts on Long Island, in the Hudson Valley and upstate New York.

Mamdani “has produced nothing and has been entirely devoted to galvanizing a small group of socialists to be his backers. That’s been his modus operandi,” Mr. Lavine said. He said he believed Mr. Mamdani harbored antisemitic views, and sensed “from him a sense of condescension and arrogance.”