>They cannot be arrested while in session.
In Article I, Section 6, Clause 1, the Framers provided for Members of Congress to be free from arrest when attending or traveling to and from Congress except in cases of treason, felony, or breaches of the peace.1 In interpreting this provision, the Supreme Court has held that the phrase "treason, felony, and breach of the peace" encompasses all criminal offenses.2 Consequently, Members are only privileged from arrests arising from civil suits, which were common in America at the time the Constitution was ratified.3
In providing for Members to "be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same,"4 the Framers followed English parliamentary and colonial practices as well as precedent established by the Articles of Confederation. The Articles provided that "the members of Congress shall be protected in their persons from arrests and imprisonments, during the time of their going to and from, and attendance on, Congress, except for treason, felony or breach of the peace."5 In his Commentaries on the Constitution of the United States, Justice Joseph Story discussed the practice of privileging members of Parliament and colonial legislatures from arrest,6 reasoning that privilege from arrest reflected the "superior duties" of members of legislative bodies to the legislative process and the representation of their constituents.7 Justice Story stated:
When a representative is withdrawn from his seat by a summons, the people whom he represents, lose their voice in debate and vote, as they do in his voluntary absence. When a senator is withdrawn by summons, his state loses half its voice in debate and vote, as it does in his voluntary absence. The enormous disparity of the evil admits of no comparison. The privilege, indeed, is deemed not merely the privilege of the member, or his constituents, but the privilege of the house also.8
Whether the provision in Article I, Section 6, excluding "Treason, Felony, and Breach of the Peace" offenses from the privilege from arrest applied to all criminal offenses or only criminal offenses involving violence and public disturbance has been subject to debate. After examining the historical meaning of the provision, the Supreme Court in Williamson v. United States, concluded that the qualifying language encompassed all criminal offenses. The Williamson Court adopted the government’s position, which was summarized by the Court as follows:
[T]he words "breach of the peace" should not be narrowly construed, but should be held to embrace substantially all crimes, and therefore as in effect confining the parliamentary privilege exclusively to arrests in civil cases. And this is based not merely upon the ordinary acceptation of the meaning of the words, but upon the contention that the words "treason, felony, and breach of the peace," as applied to parliamentary privilege, were commonly used in England prior to the Revolution, and were there well understood as excluding from the parliamentary privilege all arrests and prosecutions for criminal offenses; in other words, as confining the privilege alone to arrests in civil cases, the deduction being that when the framers of the Constitution adopted the phrase in question they necessarily must be held to have intended that it should receive its well-understood and accepted meaning.9
Consequently, under Supreme Court precedent, the privilege from arrest applies only to civil cases.10 As one commentator has noted: "In practice, since the abolition of imprisonment for debt, this particular clause has lost most of its importance."11
While the privilege prevents Members from being arrested in civil suits, it does not prevent them from being served with subpoenas. In United States v. Cooper, Thomas Cooper, a newspaper publisher, was indicted under the Sedition Act of 1798 for libeling President John Adams. Cooper sought to compel several members of Congress to testify as witnesses at his trial. In allowing Cooper to subpoena Members of Congress, Justice Samuel Chase, in a Circuit Court decision, stated: "I do not know of any privilege to exempt members of congress from the service, or the obligations of a subpoena . . . ."12 Over a hundred years later, Justice Louis Brandeis reached a similar conclusion in Long v. Ansell, holding that the privilege from arrest was limited to arrests in civil cases and did not encompass service of process. Writing for the Court, Justice Brandeis stated: "History confirms the conclusion that the immunity is limited to arrest."13
https://constitution.congress.gov/browse/essay/artI-S6-C1-2/ALDE_00013354/