Passed on May 13, 1912, the Seventeenth Amendment was the only constitutional amendment to substantially change the structure of Congress. Before, state legislators chose U.S. Senators for their state. But the Seventeenth Amendment gave the power to fill U.S. Senate seats directly to the voters.
"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."
United States Library of Congress, The Constitution of the United States of America: Analysis and Interpretation
The ratification of the Seventeenth Amendment was the outcome of increasing dissatisfaction with the originally established method of electing Senators. As more people became eligible to vote, the belief became widespread that Senators ought to be popularly elected in the same manner as Representatives. Acceptance of this idea was fostered by the mounting accumulation of evidence of the practical disadvantages and malpractices attendant upon legislative selection, such as:
Prior to ratification, however, many states had perfected arrangements calculated to afford the voters more effective control over the selection of Senators. State laws were amended so as to enable voters participating in primary elections to designate their preference for one of several party candidates for a senatorial seat, and nominations unofficially effected thereby were transmitted to the legislature. Although their action rested upon no stronger foundation than common understanding, the legislatures generally elected the winning candidate of the majority, and, indeed, in two states, candidates for legislative seats were required to promise to support, without regard to party ties, the senatorial candidate polling the most votes. As a result of such developments, at least 29 states by 1912, one year before ratification, were nominating Senators on a popular basis, and, as a consequence, the constitutional discretion of the legislatures had been reduced to little more than that retained by presidential electors.1
Very shortly after ratification, it was established that, if a person possessed the qualifications requisite for voting for a Senator, his right to vote for the Senator was not derived merely from the constitution and laws of the state that chose the Senator but from the Constitution of the United States.2 Consistent with this view, federal courts declared that, when local party authorities, acting pursuant to regulations prescribed by a party's state executive committee, refused to permit a black citizen, on account of his race, to vote in a primary to select candidates for the office of U.S. Senator, they deprived him of a right secured to him by the Constitution and laws, in violation of this Amendment.3 An Illinois statute, by contrast, that required that a petition to form, and to nominate candidates for, a new political party be signed by at least 25,000 voters from at least 50 counties was held not to impair any right under the Seventeenth Amendment, notwithstanding that 52 percent of the state's voters were residents of one county, 87 percent were residents of 49 counties, and only 13 percent resided in the 53 least populous counties.4