In Arizona v. New Mexico, 425 U. S. 794, 796 (1976), the Supreme Court declined to exercise original jurisdiction of an action by Arizona against New Mexico challenging a New Mexico electricity tax because of a pending state-court action by three Arizona utilities challenging the same tax: "We are persuaded that the pending state-court action provides an appropriate forum in which the issues tendered here may be litigated." (425 U. S., at 797.)