The United States Senate is the upper chamber of the United States Congress. Senators have been directly elected by state-wide popular vote since the Seventeenth Amendment to the United States Constitution in 1913. A senate term is six years with no term limit. Every two years a third of the seats are up for election. Some years also have a few special elections to fill vacancies. Each state has two senators elected in different years. There were 96 senators from 1912 to 1960 and 100 since then. The Senate has been dominated by Democrats (D) and Republicans (R) in the whole period.