As the House Judiciary Committee explained during the impeachment of President Nixon:
Whatever the limits of legislative power in other contexts—and whatever need may otherwise exist for preserving the confidentiality of Presidential conversations—in the context of an impeachment proceeding the balance was struck in favor of the power of inquiry when the impeachment provision was written into the Constitution.5
This conclusion echoed an early observation on the floor of the House of Representatives that the "House possessed the power of impeachment solely, and that this authority certainly implied the right to inspect every paper and transaction in any department, otherwise the power of impeachment could never be exercised with any effect."6
The House's "sole Power of Impeachment" is the mechanism provided by the Constitution to hold sitting Presidents accountable for serious misconduct. The Department of Justice has highlighted the importance of the impeachment power in justifying the Department's view that a sitting President cannot be indicted or face criminal prosecution while in office.7 The Department's position that the President is immune from prosecution has not been endorsed by Congress or the courts, but as long as the Department continues to refuse to prosecute a sitting President, Congress has a heightened responsibility to exercise its impeachment power, if necessary, to ensure that no President is "above the law."8
The Supreme Court has recognized that Congress has broad oversight authority under the Constitution to inquire about a wide array of topics, even outside the context of impeachment:
The power of inquiry has been employed by Congress throughout our history, over the whole range of the national interests concerning which Congress might legislate or decide upon due investigation not to legislate; it has similarly been utilized in determining what to appropriate from the national purse, or whether to appropriate. The scope of the power of inquiry, in short, is as penetrating and farreaching as the potential power to enact and appropriate under the Constitution.9
The Supreme Court has made clear that Congress' authority to investigate includes the authority to compel the production of information by issuing subpoenas,10 a power the House has delegated to its committees pursuant to its Constitutional authority to "determine the Rules of its Proceedings."11
The Supreme Court has affirmed that compliance with Congressional subpoenas is mandatory:
It is unquestionably the duty of all citizens to cooperate with the Congress in its efforts to obtain the facts needed for intelligent legislative action. It is their unremitting obligation to respond to subpoenas, to respect the dignity of the Congress and its committees and to testify fully with respect to matters within the province of proper investigation.12
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