Congress derives its subpoena power from its constitutional role as the legislative branch. The Supreme Court affirmed in Watkins v. United States (1957) that "the power of the Congress to conduct investigations is inherent in the legislative process." Both the House and Senate can issue subpoenas through their committees, directing individuals to appear for testimony or produce documents. Defying a congressional subpoena can result in a contempt citation. However, the president can assert executive privilege to block disclosures related to presidential communications, and the courts have generally required a balancing test between congressional need and executive privilege. When the executive branch controls DOJ, it can decline to prosecute contempt referrals, effectively nullifying the statutory contempt mechanism.