17th Amendment
One of the most common critiques of the Framers is that the government that they created was, in many ways, undemocratic. There is little doubt of this, and it is so by design. The Electoral College, by which we choose our President, is one example. The appointment of judges is another. And the selection of Senators not by the people but by the state legislatures, is yet another. The Senatorial selection system eventually became fraught with problems, with consecutive state legislatures sending different Senators to Congress, forcing the Senate to work out who was the qualified candidate, or with the selection system being corrupted by bribery and corruption. In several states, the selection of Senators was left up to the people in referenda, where the legislature approved the people's choice and sent him or her to the Senate. Articles written by early 20th-century muckrakers also provided grist for the popular-election mill.
The 17th Amendment did away with all the ambiguity with a simple premise — the Senators would be chosen by the people, just as Representatives are.