Self-interest led the house to support the 17th Amendment (17A). Popularly elected senators represented the same constituency (albeit more numerous) as popularly elected representatives. Through their senators, pre-17A states often did their duty and blocked populist proposals from the house. Without the influence of state legislatures, the house stood to gain power in congress.4
Few voices advised caution, that despite the progressives’ propaganda regarding corruption, the senate still served its Constitutional and proper purpose, to temper and cool wild proposals from the house, protect the states from federal encroachment, and provide wise counsel and circumspection of the president’s nominees and proposed treaties.
“In opposition to the Framers, late 19th century progressives promoted a new purpose and a new foundation for the senate”
The first proposals for direct election of Senators began as early as 1826, so while fixating on the Progressives is always popular sport it ignores history. And prior to 1866 there wasn’t any consistency in how various States went about selecting Senators anyway. And when it finally was changed in 1913 it was done by Amendment, which last time anyone checked is the method that the Constitution itself requires.
The whole attempt to make a sacred cow out of State legislatures selecting Senators is ridiculous. Under the Articles of Confederation that preceded the Constitution all representation was selected by State legislatures. Switching to direct elections for the House under the Constitution wasn’t one of the hot debates featuring a warning that the Senate had to retain the old method. There was no magic under the state legislature method. Sometimes legislatures were so contentious no Senators were even chosen.