The Seventeenth Amendment
When the founding fathers wrote the Constitution, they included details on the election of the Senate. Section 3, Clause 1 reads as follows:
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.
According to this clause, senators were to be chosen by state legislators rather than elected by the people, as the members of the House were. As time went by, however, this method of choosing senators became problematic. First, in some states, the legislatures were controlled by political machines. These party organizations, headed by a single boss or a small group of leaders, controlled enough votes to maintain political power, and they chose senators who did not serve the people's interests. Second, some states deadlocked over senators, leaving seats empty for months. Finally, some senate seats were seen as a "millionaire's club," because extremely wealthy senators essentially purchased their seats. None of these situations were good for the country. From 1826, there was movement for the Senate to be more directly accountable to the people.
In 1913, the Seventeenth Amendment was ratified, and included the clause "the Senate of the United States shall be composed of two Senators from each State, elected by the people thereof, for six years." People were now able to vote directly for their senators, and the senators had to serve the people's interests.