The 17th Amendment

The 17th Amendment States: “The Senate of the United States shall be composed of two Senators from each State, elected by the people thereof, for six years; and each Senator shall have one vote. The electors in each State shall have the qualifications requisite for electors of the most numerous branch of the State legislatures.
When vacancies happen in the representation of any State in the Senate, the executive authority of each State shall issue writs of election to fill such vacancies: Provided That the legislature of any State may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct.
This amendment shall not be so construed as to affect the election or term of any Senator chosen before it becomes valid as part of the Constitution.”
The 17th Amendment was ratified on April 8, 1913 and was first put into effect for the election of 1914. It amends Article 1 Section 3 of the Constitution to provide for the direct election of Senators by the people of a state rather than their election or appointment by a state legislature.
The Seventeenth Amendment is one of the They call it a "Progressive Amendment" because it was passed during the Progressive Era, with the support of the political group known as the "Progressives". The other Progressive amendments were: the 16th Amendment, which created the income tax; the 18th Amendment, which started Prohibition of alcoholic beverages; and the 19th amendment, which gave women the right to vote.
There are two reasons for why the 17th Amendment exists today: First was the problem that many state legislatures deadlocked on their selections for the Senate. The upper house and the lower house could not agree on a choice, or it was prohibitively difficult for one candidate to get an absolute majority in each house, which was required by federal law. Some states went without representation in the Senate for years as a consequence.
The second problem involved a perception that the election of senators by state legislatures made them more susceptible to corruption by special interests. The Hearst newspapers were a major force arguing this point in the early 1900s.
“From the 1870s on, citizen pressure for direct election intensified. The House of Representatives began passing "popular election" joint resolutions by the required two-thirds margins in the early 1890s. By 1902, the House had passed nineteen resolutions calling for direct election. Due to serious opposition, the bills were not even allowed on the Senate floor until 1887.
The states were active as well. Using popular referenda, the people of Oregon engineered a system to force their legislators to select whoever won the popular senatorial vote. In 1907, the Oregon legislature chose two senators within twenty minutes. This "success" resulted in other states following Oregon’s example. The states applied the most intense pressure through calls for a Constitutional Convention. Stephen Schechter, a writer who specializes in American federalism, credits the convention calls with "forcing" the Seventeenth Amendment, the only amendment compelled in this manner. Between 1893 and 1911, thirty-one of the thirty-two required states submitted applications for a convention for the purpose of obtaining the popular election of senators.
After much legislative maneuvering, a "popular election" constitutional amendment passed through both the House and the Senate, and subsequently was sent to the state legislatures in 1912. On April 8, 1913, Connecticut became the thirty-sixth state to ratify the amendment, and the direct election of senators became an official part of the Constitution. The entire ratification process took a little less than a year to complete.
The Seventeenth Amendment enjoyed widespread popular support at the time of its ratification. Pressure for the Amendment was exerted by state legislatures, the media, the House of Representatives, and the people at large. Despite claims to the contrary, the Seventeenth Amendment fundamentally altered the federal structure of government created by the Founders. The direct election of senators eliminated the states’ representation as separate entities in the national government, and in so doing it upset the precarious balance of power between the states and the federal government.”(American Secession Project)
One Case I found on the 17th Amendment was a case to get it Repealed. “In 2004, after announcing his retirement, Senator Zell Miller introduced a constitutional amendment that would repeal the Seventeenth Amendment, arguing that it gives too much power to Washington's special interests and was an attack on federalism. Alan Keyes, the veteran of unsuccessful presidential and senatorial campaigns, has also criticized the Seventeenth Amendment. At least five prominent libertarians have advocated the amendment's repeal, on the grounds that it upsets the balance of power between the federal government and state governments.” (Wiki) By repealing this amendment would mean that it would transfer the right to elect U.S. Senators from the people back to the state legislatures, as the Constitution originally provided for.