The 17th amendment, ratified in 1913, changed they way senators are elected. Prior to 1913, Senators were elected by, and were accountable to, the respective state legislatures. After 1913, senators were chosen by popular election. 

The impact of the 17th amendment should be obvious when you consider the dramatic expansion of the federal government in the mid-20th century, with the states powerless to contain it.

Resources:

Ulysses at the Mast: Democracy, Federalism, and the Sirens' Song of the Seventeenth Amendment

The Constitution

The Articles of Confederation

Federalist #39

Federalist #62

Federalist #26

Convention of States Project

Special thanks to Kerosec Productions for producing/composing/mixing/performing the theme song.