The 17th Amendment changed the way that Senators were elected but only 125 years after the ratification of the Constitution.
Article I, Section 3 of the Constitution as ratified provides for state legislatures to choose Senators (or, in an emergency, the state's governor to appoint a Senator). This was part of the Connecticut Compromise, which also cemented the two-house Congress. Members of the House of Representatives were to reflect the population at large, both in terms of election and in terms of apportionment (based on the population of a state). Members of the Senate were meant to come from the states themselves, via the state legislature. Antifederalists wary of a powerful federal government welcomed the states-legislature approach to filling the Senate.
The problem with this approach was that several times, state legislatures proved themselves either too easy to sway (meaning amenable to persuasion by deep-pocketed special interests) or too hard to budge (meaning unable to approve anyone as a Senator). Problems with both extremes convinced Congress to act, eventually.
At various times in the early part of the 19th Century, some in Congress had proposed changing the way Senators were chosen. In 1826, 1829, and 1855, members of the House proposed such an Amendment to the Constitution, but support was lacking, especially in the Senate.
As the 19th Century drew to a close, however, the idea of making Senators electable by the general public gathered a large amount of momentum.
The Senate formally investigated three Senators on charges of corruption, and one Senator in 1900 was made to remove himself after being found to have bought votes in his state legislature.
More numerous were electoral deadlocks. Some states delayed sending anyone to the Senate because the legislators couldn't agree on whom to vote for. In just one decade near the turn of the century, 46 elections from 20 states were deemed too hard to handle, with Senate seats sitting vacant for weeks or even months. In one case, a state (Delaware) didn't fill its full representation for four years.
In 1908, Oregon became the first state to pass a law mandating a popular vote for Senators. Nebraska soon followed, and a handful of states did as well. The drive for an across-the-board popular election of Senators had encompassed 31 state legislatures by 1910.
What became the 17th Amendment was approved by both houses of Congress, going to the states on May 13, 1912. In less than a year, the Amendment was the law of the land, with the ratification of the necessary three-quarters of the states being achieved in April 8, 1913 with the approval of Connecticut.
The Amendment reads:
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.