In interviews, union leaders said that the ruling would further tilt an already uneven playing field toward employers, and that it was often not a strike itself but the threat of a strike that helped unions win concessions. “Without the threat of a strike, you have little leverage in negotiations,” said Stuart Appelbaum, the president of the Retail, Wholesale and Department Store Union, which has organized successful strikes.

Mr. O’Neill’s group, the Landmark Legal Foundation, argued that a ruling against the employer could have jeopardized the labor peace that the National Labor Relations Act was enacted to assure, “placing workers and the public at risk” by essentially blessing acts of vandalism and sabotage.

Unions and workers often deliberately plan strikes to exploit employers’ vulnerability — for example, Amazon workers walked out during the holiday season — and rely on an element of surprise to maximize the economic harm they inflict, and therefore the leverage the union gains.

In the near term, unions that are contemplating strikes or already striking, such as unions representing Hollywood writers or United Parcel Service employees whose contract expires this summer, may have to take greater precautions to insulate themselves from legal liability.

Such precautions will typically weaken the impact of strikes, said Ms. Garden, the University of Minnesota professor. “You could get unions prophylactically adopting less effective tactics — things like giving advance warning about strike, which gives the employer a lot more time to hire replacement workers,” she said.