The United Auto Workers has grown in size by opening its tent to workers in white-collar professions and adopting democratic socialist stances. However, this growth has led to political friction. The U.

A. W. Region 9A branch, led by Brandon Mancilla, endorsed Claire Valdez, Darializa Avila Chevalier, and Brad Lander in their successful House primary bids in June.

As voters headed toward a polling site in New York City in early summer, a campaign volunteer made a last-minute pitch. Claire Valdez, a state assemblywoman running for an open congressional seat, was a rank-and-file union member. A prospective voter was intrigued and asked which union. Ms. Valdez belonged to the United Auto Workers.

And no, the volunteer acknowledged, she was not actually an auto worker. The U. A.

W., a large international union headquartered in Detroit, has expanded its ranks even as its core industry has shrunk, finding new life by using organizing efforts in New York City to enroll white-collar workers in private universities, museums, and public-interest law practices. In doing so, the U. A.

W. has managed to upend the political establishment in New York City, gaining clout by buoying insurgent power. The union’s regional branch was an early endorser of Zohran Mamdani’s long-shot bid for mayor over former Gov. Andrew Cuomo.

It backed Ms. Valdez, who won her congressional primary in Brooklyn and Queens against Antonio Reynoso, the Brooklyn borough president supported by most of the city’s unions. The U. A.

W. Region 9A also endorsed Darializa Avila Chevalier, a democratic socialist who upset Representative Adriano Espaillat in a district covering Upper Manhattan and parts of the Bronx. Ms. Avila Chevalier, like Ms. Valdez, is a U. A.

W. member; she joined the union through her work as an investigator for public defenders. Ms. Valdez joined in 2018, when she was a visual arts program assistant at Columbia University, a job she held for six years until being elected to the Assembly. Bringing white-collar workers like Ms. Valdez and Ms. Avila Chevalier into the fold has helped the union buck a prolonged and substantial decline in union membership across the nation. Membership rates dropped from roughly 20 percent in 1983 to 10 percent in 2025.

After swelling its ranks to 1.5 million in the 1970s, its membership dropped to 370,000 in 2023, its lowest level since 2009, according to a report filed with the U.S. Labor Department. By the end of 2025, it had risen to more than 390,000. In Region 9A, which covers eastern parts of New York, New England and Puerto Rico, only 1,000 of the region’s 35,000 active members work in the auto industry, according to Brandon Mancilla, the region’s director.

Roughly 20,000 are based in New York City, up more than 50 percent from 2022. Mr. Mancilla, 32, argued that organized labor’s diminished power helped fuel Donald J. Trump’s victory in the 2024 presidential election, as Mr. Trump capitalized on working-class Americans’ economic frustrations. Through interviews with two dozen current and former U.

A. W. members and people in other unions in New York, a picture emerged of how the union’s Northeast branch has managed to reinvent itself.