Amendment XVII

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.