The election of senators is historically very interesting.
"The Senate of the United States shall be composed of two Senators from each state, chosen by the legislature thereof for six Years; and each Senator shall have one Vote."
The idea that state legislators would appoint Senators was intended to tie state legislatures to the federal government. That seemed to work fairly well until the mid 1800s. In the mid 1850s, the Republican party was gaining in popularity and conflict was developing with the Democratic party. When states were fairly equally divided between Democrats and Republicans, state legislatures would have difficulty appointing Senators and seats would go empty. This happened in Indiana where one seat went vacant for 2 years. These hostilities in the legislatures were also playing themselves out in society. In April 1861, the Civil War broke out further dividing states. The state of Virginia actually broke into two states when Virginia joined the Confederacy and the western part of the state (West Virginia) joined the Union.
Congress passed a law regulating how and when each state would select Senators. This helped but there were still vacant seats due to deadlocks. Ultimately, states began to find ways around the requirement for senators to come from state legislatures and some states began electing senators in various ingenious ways.
In 1911, a constitutional amendment was proposed to make it legal to directly elect senators. It passed the Senate after much debate in that year. In 1912, the House debated the amendment and eventually passed it. This sent it to the states to ratify. In less than 1 year, the amendment had been ratified by 3/4 of the states and was officially adopted. The Constitution now 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 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.