17th Amendment: "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. ..."
This amendment allows for the direct election of US senators by the people. Previously senators were elected by state legislatures.
The amendment was created because too many senators were just well connected (often wealthy) people instead of qualified people. Unfortunately, the amendment perpetuated the problem it tried to fix; now senators are even more exclusively wealthy and connected, and rely on outside money to fund campaigns.