The Supreme Court doesn’t want to decide whether to hear a case that could change the way groundwater is regulated until it hears from the Trump administration.

The court’s nine justices are asking the U.S. Solicitor General to weigh in on a pair of cases that get at the question of whether groundwater falls under the purview of federal clean water law. The court set a deadline of Jan. 4 for the Solicitor General to submit his views.

The two cases, one out...