 In that case, Cooper represented the owner of a uranium mine arguing that Virginia's ban on uranium mining was preempted by federal law and was therefore unenforceable.Hollingsworth v. Perry

On March 26, 2019, Cooper argued "Hollingsworth v. Perry".The substantive question in that case was whether the Equal Protection Clause of the Fourteenth Amendment prohibits the state of California from defining marriage as the union of one man and one woman.