And that some of those who became involved in campaigns last time were questioned by state and federal investigators during the inquiries into Mr. de Blasio’s fund-raising during the 2013 campaign has also dampened some voices. (No charges were filed as a result of the investigations.)

“They’ve been dissuaded, or chilled, by some of the reaction to 2013 spending,” said Scott Levenson, the president and founder of the Advance Group, which ran an independent spending campaign against Christine C. Quinn, then the City Council speaker and the presumptive favorite for mayor. By that point, $780,000 had already been spent against her through the group known as New York City Is Not For Sale.

The backers of that effort, Wendy Neu and Stephen W. Nislick and their animal rights group, New Yorkers for Clean, Livable and Safe Streets, known as NYClass, have returned this year with a promise to spend money. But they will do so in some City Council races, not against the mayor. A similar promise has been made by the union for New York Police Department patrol officers, though no spending is expected in that effort until next month.

“We’re not looking at running against an incumbent at this time,” said Chris Coffey, of Tusk Strategies, who is representing NYClass in its 2017 efforts. “This is certainly not against the mayor. These are focused on the City Council races.” (Mr. Coffey is also representing the police union.)

Others that backed big spending last time include the Real Estate Board of New York, which supported 22 City Council candidates under the auspices of a group known as Jobs for New York. So far it is holding its fire.