Article I, Section 6, Clause 1 of the Constitution states that members of Congress ''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.'' Despite this language, courts have not interpreted this privilege to apply to criminal arrests for over a century, making it largely obsolete in modern practice.
The Founders included this provision to prevent political opponents from using arrest to interfere with legislative business, a tactic British monarchs had used against Parliament. Today, the privilege only protects members from arrest in civil suits, not criminal cases. Courts have clarified that ''breach of the peace'' is broadly defined to include nearly all criminal offenses, meaning the exceptions swallow the rule. The privilege also does not protect members from service of legal process in either civil or criminal cases. This narrow scope means members of Congress receive no special immunity from criminal prosecution simply for being legislators. A separate constitutional protection, the Speech or Debate Clause in the same section, provides broader immunity for actions taken within the legislative sphere, such as speeches, debates, and votes, but this protection does not extend to criminal acts outside legislative duties.