Unions have reason to be concerned. Membership in the Building and Construction Trades Council, a union umbrella group, has slid from around 250,000 in New York City in the 1980s to about 100,000 today.

After controlling the industry for decades, unions now receive only 20 percent of permits for new private construction and renovation jobs in the city, according to Brian Sampson, president of the New York chapter of the Associated Builders and Contractors, a nonunion trade association.

Representatives of the building trades — carpenters, bricklayers and plumbers, among other workers — counter these figures by saying their members are still employed on the largest public projects and private developments.

The conflict has been playing out in the ongoing Hudson Yards project, often described as the biggest private development in the city since Rockefeller Center. The group behind it, Related Companies, traditionally used union workers almost exclusively. It’s just the kind of project the building trades depends on.

Initially, unions supplied nearly all the construction workers for Hudson Yards. But a feud arose between late 2017 and early 2018, when Related accused the building trades of violating labor agreements, while Local 79 and other unions learned that Related would be expanding its use of nonunion firms like Trade Off.

Trade Off is the “poster child of exploitation, ” said Mike Hellstrom, president of the Mason Tenders District Council, which oversees Local 79. In 2017 and 2018, a Local 79 campaign to organize workers at Trade Off helped bring to light allegations of sexual harassment and unsafe working conditions.

This development helped to spur on the building trades’ fiery protests against Related last year. During one rally, Gov. Andrew Cuomo spoke glowingly of the building trades and workers chanted, “We just punched Related in the face.”