The chairman of the New York State Democratic Party said on Thursday that he would not support Zohran Mamdani, his party’s nominee for mayor of New York City, citing fundamental disagreements over Israel and democratic socialism.

The position puts the chairman, Jay Jacobs, directly at odds with both Democratic primary voters and Gov. Kathy Hochul, his de facto boss, who endorsed Mr. Mamdani on Sunday despite her own differences with the nominee.

Mr. Jacobs, a moderate from Long Island, was so opposed to the governor’s endorsement that he told associates in recent days that he would sooner resign as chairman than back Mr. Mamdani, a 33-year-old state lawmaker and democratic socialist, according to people who spoke with the chairman.