Congress requires a two-thirds vote to remove a member of Congress, as stated in Article 1, Section 5, Clause 2 of the U.S. Constitution. Congress members also have the right to investigate the actions of members when they may be questionable.
The Constitution allows Congress to create its own rules about disciplining and investigating members. This provision helps ensure that the other two branches of government stay out of Congress's internal affairs.
Since 1789, only 20 Congress members have been expelled. Many expulsions were during the Civil War and were related to supporting the Confederate forces. As of 2014, the most likely reason for expulsion is corruption.