For other committees, she diagrams a web of money that moved from donors associated with Mr. de Blasio into party committees in several counties, and then was spent on, or by, the candidates’ campaigns.

The report detects a level of coordination that, in her view, achieved its purpose: to frustrate both disclosure requirements and contribution limits. She cites an email from the campaign manager for a Senate candidate in Ulster County to the treasurer of the county Democratic committee. “Has the check for $60K cleared?” the campaign manager asked. “Below is our banking info, we need the 60 transferred over ASAP please.”

This email shows that they “previously had discussions about this matter,” Ms. Sugarman wrote, adding, “This pattern of activity indicates that the committees already had committed to these expenditures prior to receiving the funds.” Thus, the everyday plaintive wails of political operatives trying to win elections — I need that check ASAP! — was rendered as an act in a nearly sinister conspiracy.

Others might see this as tactical spending and fund-raising; to Ms. Sugarman, it amounts to an evasion forbidden by law. It certainly has a long history. Mr. de Blasio’s predecessor as mayor, Michael R. Bloomberg, sent $75,000 to the New York State Senate Republican Campaign Committee during the last days before a special election on Long Island in 2007. The check was delivered on Jan. 26, for an election held 11 days later. The money supported a single Republican candidate, in one campaign, with a donation many times the limit Mr. Bloomberg would have been permitted if he had given the money directly to the campaign.

Mr. Bloomberg spent heavily in similar fashion during his time as mayor to prop up the Republican Senate majority, using his own money. Mr. de Blasio has spent heavily in hopes of attaining a Democratic majority, though he has taken the precaution of using other people’s money.

If it seems strange that Mr. de Blasio is now on the griddle when so many others could just as easily have provided fixings for the same meal, it’s important to remember that the corrupting force of campaign money was part of the work of the Moreland Commission in 2013 and 2014.

The city, unlike the state, has a reasonably strong and effective public financing law. Elsewhere, money in politics finds its own level. It is not a heartening sight, said Bill Cunningham, a former executive director of the state Democratic committee.

“That level,” he said, “is somewhere between a bog and a swamp.”