But developers such as Related and Gotham Construction have argued that the labor agreements did not provide anywhere near the savings the unions had promised. And union contractors have continued to lose work to nonunion builders, especially in residential construction, although they remain dominant on large office buildings and public works.

“This has gone from one of the most highly unionized sectors of the American economy to one where huge chunks of it now operate on a nonunion basis,” said Joshua B. Freeman, a labor historian at the City University of New York. “There’s been a slow chipping away of union clout.”

Increasingly, developers who had customarily done all-union jobs have turned to “open shops,” in which union contractors compete against nonunion outfits for work.

At the Times Square rally, Bryant Nunez, a Marine veteran and a second-year apprentice with the ironworkers union, said, “The nonunion guys are taking our work. It’s pretty much a bread-and-butter issue for me.”

Related insists that its intention is not to break the unions, but to hold unions to their agreements. The company says that some have not made negotiated changes, like having work crews include more apprentices, paid at a lower wage, in order to bring costs down.

Last year, Gov. Andrew M. Cuomo sided with the construction unions, insisting that developers participating in a citywide affordable housing program must pay minimum wage rates that the unions argued would allow them to better compete for work. But it is not clear that the change has resulted in the unions’ getting more work on residential projects.

Although Related has been the most aggressive in its approach to the unions, the industry’s powerful Real Estate Board of New York has also taken up the cause. The unions, said John Banks, president of the board, have “chosen to try and apply pressure through the political process and aggressive public theatrics, rather than reform of their work rules and benefit structure. These futile efforts will result in such unions losing even more market share.”