 Cooper represented the proponents of Proposition 8, a ballot initiative adopted by the voters of California that defined marriage as being between one man and one woman.South Central Bell Telephone Company v. Alabama

On January 19, 1999, Cooper argued the case "South Central Bell Telephone Company v. Alabama".In this case, the Supreme Court considered whether Alabama's franchise tax discriminates against interstate commerce, in violation of the Commerce Clause, and whether the Alabama Supreme Court's refusal to permit the South Central Bell Telephone Company and others to raise their constitutional claims because of res judicata deprives them of the due process of law guaranteed by the Fourteenth Amendment.