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 such 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.

However, starting in roughly the 1830s and then more dramatically after the Civil War, the vision the Founders had—in which state legislatures would deliberate over the selection of Senators—began to fray. First, politicians seeking Senate seats began campaigning for state legislative candidates in a process known as the “public canvass.” The result was that state legislative races became secondary to Senate races. The most famous instance of this was the race for Senate in Illinois in 1858, in which Abraham Lincoln faced off with Stephen Douglass despite neither being on the ballot. In 1890s, many states started holding direct primaries for Senate, reducing the degree of influence state legislatures had over selection. Some states went further and began using something known as the “Oregon System,” under which state legislative candidates were required to state on the ballot whether they would abide by the results of a formally non-binding direct election for U.S. Senator. By 1908, twenty-eight of the forty-five states used the Oregon System or some other form of direct elections. 

The push for the Seventeenth Amendment occurred both in state legislatures and the House of Representatives. Between 1890 and 1905, thirty-one state legislatures passed resolutions either calling on Congress to pass an amendment providing for the direct election of senators, to hold a conference with other states to work on such an amendment, or to have a constitutional convention such that the direct elections for Senator could be included in a newly drawn Constitution. Amendments to the Constitution providing for direct elections passed the House in each session between 1893 and 1912. 

(Ironically, however, big city party machines supported the Seventeenth Amendment, largely because state legislative apportionment gave greater representation to rural areas due to districting decisions in the absence of “one person, one vote” and because machine-controlled cities could more easily mobilize voters. Many big special interests supported it as well.) William Randolph Hearst famously hired muckraking journalist David Graham Phillips to write an expose, “The Treason of the Senate,” which played a major role in debates around the Seventeenth Amendment.

Further, supporters of the Amendment argued that races for Senate swamped interest in state issues in state legislative races, reducing the accountability of state legislatures on any issue other than the identity of Senators.

The popular perception that Senate seats could be bought in backrooms of state legislatures fueled support for direct elections.