Ratified on April 8, 1913, the Seventeenth Amendment (Amendment XVII) to the United States Constitution 
changed the method by which United States Senators were elected. For the first 125 years Article 1, section 3 of the Constitution required US Senators to be elected by the state legislatures. A number of problems in the United States Senate created the need to change how Senators were elected from indirect to direct elections by the people of the United States.

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Background 
When the Constitution was adopted in 1788, Senators were to be elected by the state legislatures. This remained in effect for 125 years. By the late 19th century it became clear there were many problems with this procedure. In addition to long vacancies that lasted for months and even years, political machines had gained control over state legislatures. This corrupted the process and the Senate was seen as a "millionaire's club" who served private interests instead of the people. Beginning in the 1890s, the House of Representatives passed several resolutions for a constitutional amendment which proposed Senators be elected by direct election. The Senate refused to even take a vote on these proposals.

At this point, many states changed their strategy. Article Five of the United States Constitution offers two methods for amending the Constitution. In addition to the common method of a two-thirds vote by both houses of Congress, was another method. It stated that if two-thirds of the state legislatures apply for one, Congress must call a constitutional convention. The method had not been used before. When it appeared two-thirds of the states wanted the change, Congress acted. 

Joint resolution 39 was passed by the House of Representatives in 1911 proposing a constitutional amendment to directly elect US Senators. The Senate accepted the resolution but since it included a change, it went back to the House. The amended Joint resolution was sent to the states for ratification a year later. The required three-quarters of the states ratified the amendment and the Seventeenth Amendment became official on April 8, 1913.

Clauses

Clause 1 
The people of the United States are to elect two Senators from each state, for six years. Each Senator will have one vote in the Senate. There are 100 Senators and 100 votes. "The electors in each State shall have the qualifications requisite for electors of the most numerous branch of the State legislatures" means that anyone who can vote in a state election in their state, may also vote for a Senator. In the archaic language of the Constitution this was written in such a way to allow for states having different qualifications for persons eligible to vote for their Senator.

Clause 2 
The vacancies clause. Previously, when vacancies occurred, several state legislatures were not able to agree on who would fill the position. Before the Seventeenth Amendment, vacancies could last for months and even years in some cases. Under the Seventeenth Amendment, if a Senator dies or has to leave office, his state governor may appoint a temporary Senator until a special election can be held.

Clause 3 
This is a simple addition to prevent the passage of the Seventeenth Amendment from interrupting the election or term of any Senator chosen before the amendment was passed. The United States Senate elections, 1914 were the first nationwide popular elections for Senators.

Notes

References

Other websites 
 A Century Later: The Adoption of the 17th Amendment on Direct Election of Senators
 Thanks To The 17Th Amendment, Every Vote Will Count In Undecided Senate Races

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Elections in the United States
1910s in the United States