 This case has been called “the blockbuster separation of powers case of the Rehnquist years.” 

United States v. Winstar Corp.

On April 24, 1996, Cooper argued the case "United States v. Winstar".In that case, the Supreme Court considered the question of whether the federal government can be sued by thrifts that were sent into financial trouble when Congress changed the computation of required reserves after the Federal Home Loan Bank Board encouraged actions based on the premise that the rules would not change.This case has been described as “enormously important” and creating an “important precedent on the interpretation of Government contracts.” 

Federal Election Commission v. NRA Political Victory Fund

On October 11, 1994, Cooper argued Federal Election Commission v. NRA Political Victory Fund  This case raised various questions regarding whether portions of the Federal Election Campaign Act violated the Constitutionally mandated separation of powers.