In a statement, Mr. Klein’s campaign spokeswoman, Barbara Brancaccio, said he “has shown up and delivered for Bronx and Westchester residents.” She accused Ms. Biaggi of waging “a civil war among Democrats across the state.”

Many Democrats in the district say they became aware of the I.D.C. only after the election of President Trump.

“It became clear that one of the biggest problems at the state level was this thing called the I.D.C.,” said Rebecca Lish, an actor and mother of two who lives in Riverdale. “I realized that I had been voting for the head of the I.D.C. over and over again. You see this guy in the district and he has on a blue tie. But then he goes up to Albany and he switches to a red tie.”

Ms. Lish has since become involved in a number of progressive groups in the Bronx and beyond, and has volunteered tirelessly for Ms. Biaggi.

“I believe Alessandra Biaggi will be a stronger advocate for the issues that matter to me and my family,” said Ms. Lish, pointing to stalled bills dealing with abortion rights, single-payer health care and immigrant protections, among other things.

Mr. Klein does have his ardent supporters, particularly residents like Monique Johnson, whom he has helped in small but meaningful ways. As president of the Throggs Neck Resident Council, she is the liaison between the 1,700 residents of a public housing complex in the Bronx and the city’s Housing Authority.

In a phone interview, Ms. Johnson said Mr. Klein had been generous with his time and discretionary dollars: calling bingo numbers; running meetings about heat and hot water; donating school supplies and holiday toys; helping children get tested for lead exposure; and securing $1.5 million to overhaul a ball field.