Final answer:
The Seventeenth Amendment is the constitutional amendment that changed the election of U.S. senators from being chosen by state legislatures to a direct election by the people, which was ratified in 1913.
Step-by-step explanation:
The amendment that changed the way U.S. senators are elected is the Seventeenth Amendment. Before the enactment of the Seventeenth Amendment in 1913, senators were elected by state legislatures. This system was altered to allow for direct election of senators by the citizens, a reaction to the corrupt processes and scandals that surrounded senatorial elections during the late 1800s and early 1900s. The adoption of this amendment ensured that senators would be elected in the same manner as members of the House of Representatives - by a direct vote of the people.
Article I, Section 3 of the Constitution originally outlined the election of senators by state legislatures. However, the passage of the Seventeenth Amendment overrode this provision. The amendment addressed a number of issues including the people's dissatisfaction with the lack of direct representation as well as the desire for more democratic and transparent government processes. This significant constitutional change was ratified after it was approved by both houses of Congress and subsequently by the required number of states.
Ultimately, the Seventeenth Amendment played a critical role in shaping the democracy of the United States by giving power back to the people, allowing them to choose their senators directly. This change still stands today, with senators serving six-year terms and every state having two senators, regardless of its population.