In 2014, as Success Academy was battling with Mayor Bill de Blasio over space in the city’s public schools, Families for Excellent Schools organized a large rally in Foley Square in lower Manhattan. Karsten Moran for The New York Times

Families for Excellent Schools, a charter-schools organization known for its battles with Mayor Bill de Blasio and its close relationship with Eva S. Moskowitz, the mayor’s frequent antagonist and head of the city’s largest charter school network, Success Academy, said on Monday that it was shutting down.

The organization announced last week that it was firing Jeremiah Kittredge, its chief executive officer, after an accusation of “inappropriate behavior toward a non-employee.” But the decision to close seemed to reflect financial problems rather than the loss of a single employee.

Families for Excellent Schools for years was the well-funded face of the charter school movement in New York, but its support seems to have evaporated. The organization’s chairman, Bryan Lawrence, said in a statement: “Unfortunately, after a series of challenges over the past year and particularly given recent events, we have determined that the support necessary to keep the organization going is not there.”

In 2014, the year that Mr. de Blasio took office, Families for Excellent Schools spent $9.6 million on lobbying, more than any other organization in the state. Much of that money was spent on television advertisements criticizing the mayor for blocking several of Ms. Moskowitz’s schools from getting space in city-owned school buildings. (As a candidate, Mr. de Blasio had promised to halt a practice of giving charters space in public school buildings.)

Families for Excellent Schools also organized a rally in Albany that year of thousands of charter school students and parents. In the end, it won the fight, with Mr. de Blasio backing down and Gov. Andrew M. Cuomo and the Legislature passing major new protections for charter schools, which required the city to give them space or else cover at least part of their rent on private space.

As a 501(c) 3 organization, Families for Excellent Schools is not required under New York State law to disclose its donors.

The group ran into trouble, however, in Massachusetts, where a related organization, Families for Excellent Schools-Advocacy, spent $15 million in 2016 as part of an unsuccessful effort to expand charter schools in the state. The ballot measure it backed was overwhelmingly defeated. In the aftermath, the state’s Office of Campaign and Political Finance concluded that Families for Excellent Schools-Advocacy had violated the state’s campaign finance law and fined it $426,466, the largest fine in the history of the office. To resolve the case, Families for Excellent Schools-Advocacy agreed to dissolve, and Families for Excellent Schools agreed not to fund-raise or engage in any election-related activity in Massachusetts for four years.

Families for Excellent Schools-Advocacy was also forced to disclose its donors, which, according to Maurice Cunningham, an associate professor of political science at the University of Massachusetts Boston, may have been the biggest blow of all.

“You can only surmise they had a couple of dozen extremely angry wealthy people,” he said. “That is just not good for fund-raising.”

Last week, Mr. Lawrence released a statement saying that Mr. Kittredge had been fired. The reasons were not entirely clear. Mr. Lawrence said the organization had received a complaint about inappropriate behavior by Mr. Kittredge and had investigated it, and that Mr. Kittredge’s termination came as a result of “this investigation and additional factors.” The online news organization Politico later reported that a woman who attended a conference with Mr. Kittredge in November had accused him of sexual harassment.

Politico also first reported that the organization was closing.

A spokeswoman for Success Academy said on Monday that the charter network had ended its relationship with the group last week, "upon learning of the investigation into Jeremiah Kittredge’s actions and his termination.”

In fiscal year 2016, the organization reported revenue of more than $19 million and expenses of more than $20 million. Its website lists 40 employees.