Article I, Section 5, of the United States Constitution provides that “Each House [of Congress] may determine the Rules of its proceedings, punish its members for disorderly behavior, and, with the concurrence of two-thirds, expel a member.” According to the Senate’s Expulsion and Censure page, since 1789, the Senate has expelled only fifteen of its entire membership. Of that number, fourteen were charged with support of the Confederacy during the Civil War. In several other cases, the Senate considered expulsion proceedings but either found the member not guilty or failed to act before the member left office. In those cases, corruption was the primary cause of complaint.
According to the CRS report Recall of Legislators and the Removal of Members of Congress from Office (Jan. 5, 2012 RL30016), “While there are no specific grounds for an expulsion expressed in the Constitution, expulsion actions in both the House and the Senate have generally concerned cases of perceived disloyalty to the United States, or the conviction of a criminal statutory offense which involved abuse of one’s official position. Each house has broad authority as to the grounds, nature, timing, and procedure for an expulsion of a Member. However, policy considerations, as opposed to questions of authority, have appeared to restrain the Senate and House in the exercise of expulsion when it might be considered as infringing on the electoral process, such as when the electorate knew of the past misconduct under consideration and still elected or re-elected the Member.” — Joe