New York’s annual parade celebrating Israel has been a standard stop for the state’s politicians for the last 60 years, drawing in governors, senators and every mayor since Robert F. Wagner to pay their respects to the Jewish community.

Now, as Israel’s standing in the United States has fallen precipitously since the Gaza war, New York City Democrats have nominated a mayoral candidate who does not shy away from his record of anti-Israel activism, underlining an extraordinary departure from past mayors and from current Democratic leadership in Washington.

Assemblyman Zohran Mamdani’s victory in the city with the largest Jewish population in the world offered the starkest evidence yet that outspoken opposition to Israel and its government — and even questioning its existence as a Jewish state — is increasingly acceptable to broader swaths of the party, even in areas where pro-Israel Jews have long been a bedrock part of the Democratic coalition.

Some surveys showed Mr. Mamdani winning as many as one in five Jewish Democrats, with supporters including Brad Lander, the city’s comptroller, who also ran for mayor and encouraged his supporters to back Mr. Mamdani through a cross-endorsement. And on Wednesday, Representative Jerrold Nadler, one of the city’s most prominent Jewish leaders, endorsed Mr. Mamdani, saying they would work together “to fight against all bigotry and hate.”

But for other Jews around the country who were already struggling with their place in the progressive movement, Mr. Mamdani’s stunning victory confirmed their worst fears about the direction of the American left, fueling a sense that urgent concerns about the community’s safety are being dismissed in a movement and a city that Jews helped build.