The 17th Amendment
''Americans did not directly vote for senators for the first 125 years of the Federal Government. The Constitution, as it was adopted in 1788, stated that senators would be elected by state legislatures. The first proposal to amend the Constitution to elect senators by popular vote was introduced in the U.S. House of Representatives in 1826, but the idea did not gain considerable support until the late 19th century when several problems related to Senate elections had become evident. Several state legislatures deadlocked over the election of senators, which led to Senate vacancies lasting months and even years. In other cases, political machines gained control over state legislatures, and the Senators elected with their support were dismissed as puppets. In addition, the Senate was seen as a “millionaire's club” serving powerful private interests. The rise of the People's Party, commonly referred to as the Populist Party, added motivation for making the Senate more directly accountable to the people.
The Senate of the United States shall be composed of two Senators from each State, elected by the people thereof for six years; and each Senator shall have one vote. The electors in each State shall have the qualifications requisite for electors of the most numerous branch of the State legislatures. When vacancies happen in the representation of any State in the Senate, the executive authority of such State shall issue writs of election to fill such vacancies: Provided, That the legislature of any State may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct. This amendment shall not be so construed as to affect the election or term of any Senator chosen before it becomes valid as part of the Constitution.''