The 17th amendment to the constitution states that United States Senators will be elected by a vote of the people in the states. What else would we do? Well prior to the 17th amendment Senators were chosen by state legislators to represent the state at the federal level. They had control over the U.S. Senator in that they would even tell them how they were expected to vote on various bills.
Originally the people of the states would vote for their U.S. House representatives every 2 years, and the state legislators would appoint a U.S. Senator every 6 years. This gave the people representation in the government, but also allowed the state, as a state, to be represented.
In 1835 8 U.S. Senators were forced by their states to resign from their posts after not following the states will in their voting regarding President Andrew Jackson's war against the Bank of the United States. So you can see that consequences to Senators under this system are swift and severe. Contrast that with today where U.S. Senators only face voters every 6th year.
The 17th amendment was ratified in 1912. It took more than 85 years to become law since the first time it was introduced.