Article I, Section 6, Clause 1:
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
Until recently, the Court distinguished between Members of Congress, who were immune from suit arising out of their legislative activities, and legislative employees who participate in the same activities under the direction of a Member. 1 Thus, in Kilbourn v. Thompson, 2 the sergeant at arms of the House was held liable for false imprisonment because he executed the resolution ordering Kilbourn arrested and imprisoned. Dombrowski v. Eastland 3 held that a subcommittee counsel might be liable in damages for actions as to which the chairman of the committee was immune from suit. And, in Powell v. McCormack, 4 the Court held that the presence of House of Representative employees as defendants in a suit for declaratory judgment gave the federal courts jurisdiction to review the propriety of the plaintiff's exclusion from office by vote of the House.
Upon full consideration of the question, however, the Court, in Gravel v. United States, 5 accepted a series of contentions urged upon it not only by the individual Senator but by the Senate itself appearing by counsel as amicus:
that it is literally impossible, in view of the complexities of the modern legislative process, with Congress almost constantly in session and matters of legislative concern constantly proliferating, for Members of Congress to perform their legislative tasks without the help of aides and assistants; that the day-to-day work of such aides is so critical to the Members’ performance that they must be treated as the latter's alter ego; and that if they are not so recognized, the central role of the Speech or Debate Clause . . . will inevitably be diminished and frustrated. 6 Therefore, the Court held
that the Speech or Debate Clause 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. 7
The Gravel holding, however, does not so much extend congressional immunity to employees as it narrows the actual immunity available to both aides and Members in some important respects. Thus, the Court said, the legislators in Kilbourn were immune because adoption of the resolution was clearly a legislative act but the execution of the resolution – the arrest and detention – was not a legislative act immune from liability, so that the House officer was in fact liable as would have been any Member who had executed it. 8 Dombrowski was interpreted as having held that no evidence implicated the Senator involved, whereas the committee counsel had been accused of
conspiring to violate the constitutional rights of private parties. Unlawful conduct of this kind the Speech or Debate Clause simply did not immunize. 9 And Powell was interpreted as simply holding that voting to exclude plaintiff, which was all the House defendants had done, was a legislative act immune from Member liability but not from judicial inquiry.
None of these three cases adopted the simple proposition that immunity was unavailable to House or committee employees because they were not Representatives or Senators; rather, immunity was unavailable because they engaged in illegal conduct that was not entitled to Speech or Debate Clause protection. . . . [N]o prior case has held that Members of Congress would be immune if they executed an invalid resolution by themselves carrying out an illegal arrest, or if, in order to secure information for a hearing, themselves seized the property or invaded the privacy of a citizen. Neither they nor their aides should be immune from liability or questioning in such circumstances. 10