When Richard Nixon tried to withhold evidence from the Watergate trial, what power of the president did he claim allowed him to do that?

United States v. Nixon, 418 U.S. 683 (1974), was a landmark United States Supreme Court case that resulted in a unanimous decision against President Richard Nixon, ordering him to deliver tape recordings and other subpoenaed materials to a federal district court. Issued on July 24, 1974, the decision was important to the late stages of the Watergate scandal, when there was an ongoing impeachment process against Richard Nixon. United States v. Nixon is considered a crucial precedent limiting the power of any U.S. president to claim executive privilege.