The Constitution, as it was adopted in 1788, made the Senate an assembly where the states would have equal representation. Each state legislature would elect two senators to 6-year terms. Late in the 19th century, some state legislatures deadlocked over the election of a senator when different parties controlled different houses, and Senate vacancies could last months or years. In other cases, special interests or political machines gained control over the state legislature. Progressive reformers dismissed individuals elected by such legislatures as puppets and the Senate as a "millionaire’s club" serving powerful private interests.
One Progressive response to these concerns was the "Oregon system," which utilized a state primary election to identify the voters’ choice for Senator while pledging all candidates for the state legislature to honor the primary’s result. Over half of the states adopted the "Oregon system," but the 1912 Senate investigation of bribery and corruption in the election of Illinois Senator William Lorimer indicated that only a constitutional amendment mandating the direct election of Senators by a state’s citizenry would allay public demands for reform.
When the House passed proposed amendments for the direct election of Senators in 1910 and 1911, they included a "race rider" meant to bar Federal intervention in cases of racial discrimination among voters. This would be done by vesting complete control of Senate elections in state governments. A substitute amendment by Senator Joseph L. Bristow of Kansas provided for the direct election of Senators without the "race rider." It was adopted by the Senate on a close vote before the proposed constitutional amendment itself passed the Senate. Over a year later, the House accepted the change, and on April 8, 1913, the resolution became the 17th amendment.
For more information, visit the National Archives' Treasures of Congress online exhibit.