The chasm in the state’s Democratic Party between the old guard and the new progressives exploded with Ms. Ocasio-Cortez’s stunning primary victory over Representative Joseph Crowley, the No. 4 Democrat in the House. It was reinforced when six Democratic challengers toppled Senate incumbents who had belonged to the Independent Democratic Conference, a group of Democrats who had collaborated with Republicans in Albany.

[Read more about the I.D.C.’s impact on progressive politics.]

Still, in Brooklyn, progressives argue that proxy cards continue to prop up the establishment.

Last week, however, Mr. Seddio came under fire after his team sent out misleading fliers. The letters urged committee members to turn over their votes to Mr. Seddio, whose name was preprinted on the proxy, and included the names of elected officials supportive of the effort. But Mr. Seddio never got permission to include some of the officials’ names — some of which were spelled wrong.

“The level of incompetence in trying to be unethical was ridiculous,” said Doug Schneider, a district leader in the 44th Assembly District, whose name was misspelled. Mr. Schneider never gave his support to Mr. Seddio’s effort.

Mr. Seddio apologized, saying his staff was hurried in sending the mailer. And he argued that the proxy cards are necessary to establish a quorum, pointing to the many older members of the committee who have trouble attending meetings.

But Mr. Schneider said it perpetuates control.

“The way the leadership of the Brooklyn Democratic Party has maintained control for all these years is by having people on the county committee that don’t have much interest in the county committee, which allows the county boss to have total control,” Mr. Schneider said.

Similar party dynamics are at play in Queens, where The New York Times found that candidates running for 21 seats were registered to run for the Democratic County Committee without their knowledge. Most had never heard of the committee. Yet at the same time, candidates who had hoped to join were tossed off the ballot on technicalities such as improper paperwork.