Upon taking office, senators-elect must swear or affirm that they will "support and defend the Constitution." The president of the Senate or a surrogate administers the oath to newly elected or re-elected senators. The oath is required by the Constitution; the wording is prescribed by law.
Text of the Senate's Oath
Essays on the Oath of Office
The Senate's First Act--the Oath Act, May 5, 1789
Senator Resigns to Protest Loyalty Oath, January 29, 1864
Related Items