The right of legislators to speak their minds with impunity while engaged in legislative work was acknowledged by the British Bill of Rights of 1689, written into the Articles of Confederation, and, after the Revolution, guaranteed by state constitutions as well as by the Speech and Debate Clause. James Wilson, one of the principal architects of the Constitution, explained in his Lecture on Law (1791) the purpose of the clause:

In order to enable and encourage a representative of the publick to discharge his publick trust with firmness and success, it is indispensably necessary, that he should enjoy the fullest liberty of speech, and that he should be protected from the resentment of every one, however powerful, to whom the exercise of that liberty may occasion offence.

In his Commentaries on the Constitution of the United States (1833), Justice Joseph Story wrote that in England the privilege was “strictly confined to things done in the course of parliamentary proceedings, and [did] not cover things done beyond the place and limits of duty.” To illustrate this limitation, he noted that although a libelous speech delivered in the House of Commons was privileged, if a Member republished that speech elsewhere, the libeled party was free to bring him to court. He then added that “the same principles seem applicable to the privilege of debate and speech in congress.”

Although the only early case to deal with the privilege was concerned with a virtually identical provision of a state constitution, rather than the Speech and Debate Clause itself, the Massachusetts Supreme Court agreed that the privilege was limited to actions taken by a legislator “in the exercise of the functions of [his] office.” Coffin v. Coffin (1808). This view of the scope of the privilege is consistent with that of another delegate to the Constitutional Convention, Charles Pinckney, who later observed in remarks in the U.S. Senate that the Framers “knew that in free countries very few privileges were necessary for the undisturbed exercise of legislative duties . . . they therefore not only intended, but did confine their privileges within the narrow limits mentioned in the Constitution.”

Over the past fifty years, the Supreme Court has reaffirmed that the purpose of the clause is to protect the independence of Congress when exercising the legislative responsibilities assigned to it by the Constitution, Eastland v. United States Servicemen’s Fund (1975); and it will interpret the clause broadly to that effect. United States v. Johnson (1966). The Court has also consistently limited its application to activities that are “clearly a part of the legislative process.” United States v. Brewster (1972).

An activity is deemed to be within the legislative sphere only if it is “an integral part of the deliberative and communicative processes by which Members participate in committee and House proceedings with respect to the consideration and passage or rejection of proposed legislation or with respect to other matters which the Constitution places within the jurisdiction of either House.” Gravel v. United States (1972). Thus, the Court has held that the clause protects such acts as voting, the conduct of committee hearings, the issuance and distribution of committee reports, the subpoenaing of information required in the course of congressional investigations, and even the reading of stolen classified materials into a subcommittee’s public record. Doe v. McMillan (1973). Conversely, speech and debate immunity will not protect Members engaged (even in their official capacities) in such non-legislative activities as negotiations with federal agencies, the issuance of press releases and newsletters, and the delivery of speeches in their home districts. Gravel v. United States; Hutchinson v. Proxmire (1979).

If a Member’s actions meet the “legislative process” test, his immunity is absolute; and that is so even if he has acted contrary to law. Accordingly, although the government may prosecute a Member for a criminal act, such as accepting a bribe in exchange for a vote, it may not pursue the case if proof of the crime “depend[s] on his legislative acts or his motive for performing them.” United States v. Brewster. Thus, the government may not prove that the Member voted a particular way on the House floor in exchange for a bribe; the government, however, may prove (by other means) that the Member promised to vote a particular way in exchange for the bribe. The former (the vote) requires proof of what happened on the House floor whereas the latter (the promise to vote) does not.

Members must be shielded not only from the consequences of litigation, but from its burdens because engagement in litigation of any kind “creates a distraction and forces Members to divert their time, energy, and attention from their legislative tasks.” Eastland v. United States Servicemen’s Fund. Consequently, a Member may immediately appeal a trial court’s denial of a Member’s motion to dismiss a case based on a claim of speech and debate immunity so that the Member may be spared the burden of a trial if his motion proves to be valid. Helstoski v. Meanor (1979). Circuit courts are divided on the extent of the distraction that is prohibited by the clause. The District of Columbia Circuit put a stop to the FBI’s non-consensual search of a Member’s files containing legislative and non-legislative materials because such a search will “disrupt the legislative process” irrespective of the use to which such documents may be put. United States v. Rayburn House Office Bldg. (2007). The Ninth Circuit disagrees, however, and it has ruled that the clause permits such searches. United States v. Renzi (2011).

A circuit split also exists with respect to informally gathered information, such as that secured in meetings with constituents. The Tenth Circuit holds that such information is unprotected because it is not gathered “in the course of a formal committee action.” Bastien v. Office of Campbell (2004). The Third and Ninth Circuits, however, conclude that such fact-finding is essential to the legislative process and therefore it is covered by the clause. Gov’t of Virgin Islands v. Lee (1985); Miller v. Transamerican Press, Inc. (1983). The Ninth Circuit notes in Renzi that the clause’s protection does not apply where the fact-finding involves criminal activities.

The limitations on the protections accorded Members of Congress by the Speech and Debate Clause prompted one Member also to assert the much broader privilege of “qualified immunity” from a common law defamation suit for activities he engaged in “within the scope” of his legislative duties. The D.C. Circuit, however, declined to accord Members of Congress the same kind of immunity for official acts as is enjoyed by members of the executive branch. Chastain v. Sundquist (1987).

Although the Speech and Debate Clause speaks only of “Senators and Representatives,” in order to effect its purpose the Court in Gravel declared that it applies “not only to a Member but also to his aides insofar as the conduct of the latter would be a protected legislative act if performed by the Member himself.” An aide who carries out congressional instructions that are found to be unlawful, however, is responsible for his acts even though the legislators who issued the instructions continue to be protected. Powell v. McCormack (1969); Doe v. McMillan.

In Eastland, the Supreme Court acknowledged that the clause may shield Members from civil or criminal liability “even though their conduct, if performed in other than legislative contexts, would in itself be unconstitutional or otherwise contrary to criminal or civil statutes.” The risk of such abuse, however, “was the conscious choice of the Framers’ buttressed and justified by history.” While state legislators may receive similar protection under state constitutions or common law, the clause does not protect them from prosecution for the commission of federal crimes. United States v. Gillock (1980). Errant Members nevertheless remain subject to disciplinary action by their respective houses for “disorderly behavior”—and, of course, by their constituents on election day.