The seventeenth Amendment transferred the election of Federal Senators from the State Senates directly to the people.
A survey of relevant historical conditions reveals the rational for this action: A deeply corrupt political system with gerrymandered electoral districts, with strong party bias in each district, ensuring safe seats. With Federal senators elected through the State Senate, Federal senators were effectively insulated from popular opinion.
On its face, this seems like a good and reasonable thing. However, it has been argued that a proliferation of elections is both dangerous and harmful.
I am always impressed by the thoughtfulness of the American Constitution. The modern conception of 'separation of powers' learned in junior high civics classes (Executive, legislative, judicial) was not the one that the constitutions framers were concerned with. Indeed, its worth realizing that the third member of our 'powers' triumvirate was not constitutionally created, but rather a product of Marbury vs. Madison, a court ruling that first declared a legislative action unconstitutional.
The separation of powers the constitutional framers were interested in was the one between the People, the States, and the Federal Government. The balance struck displays and interesting philosophical approach: To the people, all privileges and liberties (aka Rights)--to the Federal Government, certain restricted powers. To the States, all other legislative authority.
The only means of redress the U.S. constitution directly provides to the people is voting. While most of the State constitutions provide opportunities for petition and referendum, it is notably absent in the Federal Constitution.
The argument that Federal senators were intended to be the representative of each STATE to the Federal Government, rather then the directly elected representatives of the people, seems sound. The role of directly elected representative was already filled by the House of Representatives.
The 17th amendment, by breaking this crucial link between the States and the Federal Government, removed State oversight of Federal Legislation. It became possible for the Federal Government to interact directly with political constituencies not represented in State Government.
On one level, it is pleasing that citizens previously 'shut out' now had an increased interest in the political process. On the other hand, there may be a reason that marginal interests are not represented at the State level - marginal appeal.
This suggests a rationale for the modern Federal Governments fondness for non-dominant political interests--both trans-State organizations and ideological extremists. A national constituency can muster resources from multiple locations and then concentrate them on a single candidate in a single race. Similar to the concentration of forces in warfare, it is an effective strategy.