The United States Senate is a branch of the U.S. Congress. The Vice President is the president of the Senate, although he is not a senator. Several Presidents of the United States, including John Keeler, David Palmer, and Allison Taylor, were senators before they were sworn in to the White House.
The Senate has the power to conduct governmental investigations. One such investigation, which was televised live from Russell Senate Office Building, was conducted by Senator Blaine Mayer (along with James Gilbert, Richard Horton, and others) during Day 7. They questioned witnesses in an attempt to establish that the Counter Terrorist Unit had committed acts which were violations of human rights. One of the witnesses they subpoenaed was Jack Bauer, whose senate questioning was interrupted by the FBI.