The Speech or Debate Clause of the U.S. Constitution provides that "for any speech or debate in either House, [Senators and Representatives] shall not be questioned in any other place" The original purpose of the Clause was to protect the independence and integrity of Congress, allowing Members the freedom of speech, debate, and deliberation without fear of intimidation by the executive branch or the judiciary. Members always remain accountable to the house of Congress in which they serve, since the Clause only prohibits "questioning" outside of Congress. The Clause only protects Members, and in some cases congressional staff, when they are participating in "legislative acts." When it applies, the Clause provides immunity from civil and criminal suits and confers a testimonial privilege, which prevents compelled testimony or the use of evidence related to a protected act.
The U.S. Court of Appeals for the District of Columbia Circuit has interpreted the Clause to prohibit not only the use of privileged evidence in a court proceeding, but also the disclosure of evidence obtained through a search that permits executive branch officers to review privileged materials without a Member's consent. In United States v. Rayburn House Office Building, Representative William Jefferson asserted that a search of his office, conducted by the FBI pursuant to a search warrant relating to a bribery investigation, violated the Speech or Debate Clause. The D.C. Circuit agreed, holding that search procedures that deny a Congressman "any opportunity to identify and assert the privilege with respect to legislative materials" and instead "allow agents of the Executive to review privileged materials without the Member's consent violates the Clause." In reaching its decision, the court emphasized that the Clause was meant to prevent intrusions into the legislative process and preserve legislative independence, holding that "compelled disclosure [pursuant to a search] may  chill the exchange of views with respect to legislative activity. This chill runs counter to the Clause's purpose of protecting against disruption of the legislative process." The court made clear however, that the Clause does not prohibit "inquiry into illegal conduct simply because it has some nexus to legislative functions."
While the D.C. Circuit in Rayburn concluded that the Clause provides Members with protection not only from use of privileged evidence against them, but also from disclosure of such evidence, the Ninth Circuit appears to disagree. In United States v. Renzi, the appeals court confronted this question in the context of a bribery prosecution against a former Member. The Renzi court explicitly disagreed with the Rayburn court's interpretation of the Clause, holding that the Clause only provides a non-disclosure privilege (and prevents the use of evidence derived from privileged information) when the underlying act being investigated is a legislative act protected by the Clause. Therefore, the court concluded that Members, such as Representatives Renzi and Jefferson, are not entitled to a non-disclosure privilege because the underlying acts being investigated, namely bribery, were not legislative acts covered by the Clause. The Ninth Circuit also noted that the U.S. Supreme Court has never adopted the expansive interpretation of the privilege announced in Rayburn.
- Congressional Research Service, "Who's Overseeing Whom? The CIA, SSCI, and the Speech or Debate Clause" (Mar. 13, 2014) (full-text).