Final answer:
The 17th Amendment to the United States Constitution establishes the direct election of U.S. Senators by popular vote.
Step-by-step explanation:
The U.S. Amendment that establishes the direct election of U.S. Senators by popular vote is the 17th Amendment to the United States Constitution. Before this amendment, senators were appointed by state legislatures.
The passage of the 17th Amendment in 1913 changed the selection process, allowing senators to be elected directly by the people of the United States.
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