The Supreme Court has granted a stay of a final rule issued by the Environmental Protection Agency. The rule aims to reduce carbon dioxide emissions from power plants fueled by fossil sources. It has the dull title, “Carbon Pollution Emission Guidelines for Existing Stationary Sources: Electric Utility Generating Units,” and is known more simply as the Clean Power Plan. Twenty-nine states and state agencies sued, claiming that the plan is illegal. The stay, by a vote of 5-to-4, halts implementation of the plan until such time as the legal challenges to it have been resolved.

The plan is now before the federal court of appeals for the District of Columbia, with a hearing scheduled for June 2. But whichever side loses in that court is almost certain to appeal to the Supreme Court. In the most likely scenario, the Justices will take the case in its 2016-17 term and decide it near the end of the term. President Obama thus would leave office before the Court decided whether his most important environmental initiative is lawful.