The key question in determining whether a company, like a fast-food corporation, is a joint employer of workers employed by another company, like one of the chain’s franchisees, is the degree of control exercised by the corporation over workers at the franchise. The ruling on Thursday declared that such control must be direct.

Under the Obama-era doctrine, the fast-food corporation could be held liable for labor violations that occurred at the franchise even if the control it exerted was indirect — for example, if it required the franchisee to use software dictating certain scheduling practices — or if it had the right to exercise control over workers that it nonetheless didn’t exercise.

The reversal could have important implications for the ability of workers to win concessions from employers through collective bargaining. In many cases, a contractor or franchisee has such low profit margins that it could not afford to raise wages or improve benefits even if it wanted to.

But when, as was more likely under the Obama-era doctrine, a wealthier company employing a contractor or conferring a franchise is considered a joint employer, it must join the bargaining and could in principle compensate workers more generously.

The reversal could also affect the ability to unionize in the first place. A company is free to fire a contractor or end a franchise arrangement if it suspects that workers are on the verge of unionizing. But there could be legal liability for doing so if the company is a joint employer of workers with the contractor or franchisee.

Employers have been so concerned about the more sweeping joint-employer standard that they have lobbied Congress to change the standard through legislation, a version of which the House passed in November. They are likely to continue to push for such legislation for fear that a future labor board under Democratic control could simply reverse the standard again, and because there are applications of the joint employer concept — as in enforcement of minimum-wage laws — not covered by the labor board’s decision.

The case before the board appeared relatively straightforward, involving two nominally separate construction companies in Iowa, Brandt Construction Company and Hy-Brand Industrial Contractors, owned by the same four people. In 2015, two employees of Brandt and five employees of Hy-Brand went on strike to highlight safety concerns and the level of their pay and benefits, and the ownership fired all seven in retaliation.