Delaying a rule describing the scope of the nation’s water pollution law in the next 25 days is “unlikely” as the Trump administration scrambles to withdraw the Obama-era regulation.

The U.S. Supreme Court will issue by Feb. 16 the mandate for its Jan. 22 unanimous decision directing federal district courts—rather than a single federal appeals court—to hear challenges to the currently stayed 2015 Clean Water Rule, and to dismiss all litigation pending against this regulation at the U.S. Court...