The Seventeenth, of course.

The 17th Amendment, which became a part of the Constitution in 1913, established the popular election of senators. Before the passage of the 17th Amendment, Senators were elected by state legislatures. In the case of a vacancy (whether by death or resignation), the state legislature selected the replacement. As a result of the 17th amendment, senators are now eleced (much like the membership of the House of Representatives) through direct election and governors have the sole power to select replacement senators.

This very much went against what the founders attended. The House was meant to speak for the people while the Senate was meant to represent the interests of the state. Before the passage of the 17th Amendment, senators were meant to be elder statesmen, scholars and politicians who had earned the right to sit in the greatest deliberative body in the world. After the passage of the 17th Amendment, the Senate came to be populated by professional politicians. The requirement for being a member of the Senate went from being a statesmen to being able to wage a winning campaign.

And that is why the U.S. Senate, which was once populated by men like Daniel Webster, Henry Clay, and Henry Cabot Lodge, is now populated by ambitious carnival barkers who are more concerned about their next election than what's good for the country. It's also why governors are now absurdly powerful. Every time that a governor is caught selling a senate appointment, you have the 17th Amendment to thank.

If you truly want to reform the Senate, don't waste your time with counter-productive measures like changing the number of senators that each state receives, which is a disgustingly elitist idea that goes against everything that makes America is supposed to stand for. Instead, repeal the 17th Amendment and return the power to the state legislatures.