I like how it discusses how the 17th amendment made special interest groups more powerful because the direct election of Senators increased their tenure (the likelihood of being reelected), which resulted in the greater chance that their promises would be kept because special interests could rely on their being there from term to term.
The seniority system made the "promise" of senior senators worth more than others, so the longer one was there the greater that their promises had value to the special interests.
-PJ
As they are wont to do, liberals exaggerated the problems with a senate of the states. A hundred years of experience with the 17th proves the Framers got it right.