On April 8, 1913, the 17th Amendment was ratified. The Amendment addresses the election of senators to Congress. It provides for the people to elect new senators (versus the state legislatures). The Amendment states–
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 Amendment goes on to clarify what happens in the event a senator doesn’t complete his or her term in office. View the original document at http://www.ourdocuments.gov/doc.php?doc=58.