When the United States Constitution was originally drafted, the Senate was designed to be a counterbalance to the influence of popular opinion. While the House was to be elected directly by the people and the representatives would serve two-year terms, the Senate was to be chosen, two in each state, by the state legislature for six-year terms. The idea was for the Senate to be "above the fray" and capable of more detached consideration of issues.
In practice, selection of senators became influenced by business interests and political machines. The Progressive Movement opposed this system and proposed to replace it with direct election by the people. Congress passed an amendment to this effect in 1912 and in 1913 it was ratified as the Seventeenth Amendment.
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 (text).