Local union leaders say the Supreme Court’s ruling on Wednesday used the First Amendment as a pretext to advance the longstanding conservative goal of weakening unions.

The 5-4 ruling forbids public-employee contracts from requiring workers to pay union dues.

The Janus ruling could weaken workers’ voices in the face of powerful corporations, said Sparrow Tobin, a board member representing Orange and Rockland counties for New York State United Teachers.

Anti-union forces are “cutting our legs out from under us,” Tobin said. “It’s ironic that unions used to give workers a voice, and (the Supreme Court is) using a First Amendment argument to suppress the voice of workers.”

The Janus decision could significantly sap resources from unions in New York and more than 20 states, according to the Empire Center, a nonpartisan New York think tank.

New York has the nation’s most heavily unionized public sector, with 1.2 million of 1.4 million government employees in unions, according to the Empire Center.

The think tank estimates that all the New York public-sector workers opting out of union membership could cost unions $110 million in revenue. All told, New York’s public-sector unions collect at least $862 million annually in dues and fees, according to the Empire Center.

In a statement, CSEA Southern Region President Billy Riccaldo, who represents the mid-Hudson, said, “Our members have been clear that this case has nothing to do with benefiting them and everything to do with weakening unions, tipping the balance in favor of the employer, and silencing working people.”

Ron Diaz, president of the Hudson Valley Area Labor Federation, said “the Supreme Court ruled in favor of big money” because the plaintiff was backed by the anti-union, conservative donor-funded National Right to Work Committee. Unions typically support Democrats.

“Do I think (the Janus decision) will weaken the union? No. CSEA is best when the pressure is on,” Adamo said. “Our membership knows what we have, what we’ve gotten, and that now more than ever we need to stick together.”