The 17th amendment of the United States constitution is the amendment that dictates the composition of the U.S. senate. It also sets out the requirement of having senators elected by popular election rather than appointment.
The framers of the constitution originally authorized state legislatures to choose Senators for their states. The rational behind this provision was that the House of Representatives should represent the interests of the people while the interests of the state are represented by the senate.
The 17th amendment changed that, requiring senators are elected by the people in a popular vote.
The amendment also provides the ability for the state legislature to empower the executive to appoint a senator in the interim when a senate seat becomes vacant, until a new election is held and the people elect a substantive senator. (Read the full text of the 17th amendment below)
Year of ratification: 1913
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.