Americans did not directly vote for senators for the first 125 years of the Federal Government. The Constitution stated that senators would be elected by state legislatures. By the late 19th century several problems related to Senate elections had become evident. Several state legislatures deadlocked over the election of senators, with no one elected to fill the Senate seat. The Senate was seen as a “millionaire's club” serving powerful private interests. The rise of the Populist Party added motivation for making the Senate more directly accountable to the people.

During the 1890s, the House of Representatives passed several resolutions proposing a constitutional amendment for the direct election of senators. Each time, however, the Senate refused to even take a vote. When it seemed unlikely that both houses of Congress would pass legislation proposing an amendment for direct election, many states changed strategies. Article V of the Constitution states that Congress must call a constitutional convention for proposing amendments when two-thirds of the state legislatures apply for one. Although the method had never previously been used, many states began sending Congress applications for conventions. As the number of applications neared the two-thirds bar, Congress finally acted.

In 1911, the House of Representatives passed a resolution proposing a constitutional amendment for direct election of senators. Over a year later, the amendment was sent to the states for ratification. On April 8, 1913, three-quarters of the states had ratified the proposed amendment, and it was officially included as the 17th Amendment.

Source: 17th Amendment to the U.S. Constitution: Direct Election of U.S. Senators
The U.S. National Archives and Records Administration