Final answer:
The Seventeenth Amendment, ratified in 1913, established the direct election of U.S. Senators by the people of the states, replacing the prior practice where state legislatures selected Senators.
Step-by-step explanation:
The constitutional amendment that passed during the Progressive Era, which allowed for the popular or direct election of U.S. Senators, is the Seventeenth Amendment. The Seventeenth Amendment to the United States Constitution was ratified in April 1913, and it mandated the direct election of U.S. senators by the people of the states. Prior to this amendment, Senators were chosen by state legislatures, but reform movements including those by the Progressive and Populist parties, as well as a series of controversial and scandalous elections, highlighted the need for change. The push for this amendment was also a part of the wider Progressive reforms that sought to increase citizen participation in the electoral process and make the government more accountable to the people.