A progressive challenger leading her first campaign was set to beat the four-term Democratic mayor of Buffalo on Tuesday in a primary upheaval that would upend the political landscape of New York City’s second largest city and signal the strength of the party’s left wing .

The challenger, India B. Walton, is a former nurse and community activist who ran with the support of the Democratic Socialists of America and the Working Families Party. She led longtime Democratic establishment Byron Brown by 7 percentage points, or about 1,500 votes, at midnight with all in-person ballots counted, unofficial results show.

If Mrs Walton, 38, won the primary and then triumphed in the November general election – a likely outcome in strongly Democratic Buffalo – she would be the first socialist mayor of a major American city since 1960, when Frank P. Zeidler resigned as as the mayor of Milwaukee. She would also be the first female mayor in Buffalo history.

Ms Walton celebrated her victory in a jubilant appeal to her mother who was captured On video, shouting, “Mom, I won. Mom, I’m the mayor of Buffalo. Well, not until January, but, yeah.

Mr Brown, who once led the state’s Democratic Party and is a close ally of Governor Andrew M. Cuomo, refused to give in despite the margin between him and Ms Walton.

“We will make sure every vote is counted,” he said. (Ms Walton’s campaign estimated that there were around 1,500 mail-in ballots outstanding.)

Ms Walton showed no hesitation in declaring victory, highlighting what she said were the national ramifications of the race. She said the mind-boggling result “would resonate here in Buffalo and across the country, showing that a progressive platform that puts people before profit is both viable and necessary.”

“Tonight’s result proves that the Buffalonians demand community-focused, people-centered government, and we stand ready to serve them,” Ms. Walton said in a statement. “For too long we have seen our city working for politicians, for developers, for the police union, but not for ordinary working families. In our city, everyone will have their place at the table.

Ms Walton said her priorities as mayor would include adopting so-called sanctuary city rules to protect undocumented immigrants, introducing stronger protections for tenants and ending the role of police in most appeals. mental health emergency.

She also criticized Mr Byron’s handling of the coronavirus pandemic and helped lead protests in the city last year against the murder of George Floyd by police.

Mr Brown, 62, did not campaign vigorously, according to his opponents, and he refused to debate Ms Walton. He has appeared regularly with Mr. Cuomo at the governor’s press conferences in western New York to promote the state’s economic reopening.

Ms Walton, in turn, relied on an intense grassroots organizing operation, a formidable fundraising effort, and the support of some of the governor’s most vocal enemies, including the Working Families Party and Cynthia Nixon. , who waged an unsuccessful primary campaign against Mr. Cuomo in 2018.