The seventeenth amendment to the constitution was a key progressive reform designed to

Make senators directly elected and end the Senate's millionaire's club.

Explanation:

Initially, the U. S Constitution provided that the members of the Senate would be elected directly by state legislatures. However, with time, different political interest groups gained control over the state legislatures.

In response to this, reformers started to question on the viability of such procedure of electing senates by powerful state legislatures. Therefore, 17th Amendment to the U. S. Constitution was passed on April 8, 1913 which mandated direct election of Senators by the citizens of the states. It invalidated state legislature's power to elect state senators and aimed to end Senate's millionaire's club who were serving rich private interests.