Severing senators from state legislatures, which could monitor and even instruct them, made them more susceptible to influence by nationally organized interest groups based in Washington. Many of those groups, who preferred one-stop shopping in Washington to currying favors in all the state capitals, campaigned for the 17th Amendment. So did urban political machines, which were then organizing an uninformed electorate swollen by immigrants. Alliances between such interests and senators led to a lengthening of the senators' tenures.
The Framers gave the three political components of the federal government (the House, Senate and presidency) different electors (the people, the state legislatures and the Electoral College as originally intended) to reinforce the principle of separation of powers, by which government is checked and balanced.
Although liberals give lip service to "diversity," they often treat federalism as an annoying impediment to their drive for uniformity. Feingold, who is proud that Wisconsin is one of only four states that clearly require special elections of replacement senators in all circumstances, wants to impose Wisconsin's preference on the other 46. Yes, he acknowledges, they could each choose to pass laws like Wisconsin's, but doing this "state by state would be a long and difficult process." Pluralism is so tediously time-consuming.
Irony alert: Feingold's amendment requiring elections to fill Senate vacancies will owe any traction it gains to Senate Democrats' opposition to an election to choose a replacement for Barack Obama. That opposition led to the ongoing Blagojevich-Burris fiasco.
By restricting the financing of political advocacy, the McCain-Feingold speech-rationing law empowers the government to regulate the quantity, timing and content of political speech. Thanks to Feingold, McCain and others, the First Amendment now, in effect, reads: "Congress shall make no law ... abridging the freedom of speech unless it really, really wants to in order to guarantee that there will be only as much speech about the government as the government considers appropriate, and at times the government approves."
Now Feingold proposes to traduce federalism and nudge the Senate still further away from the nature and function the Framers favored. He is, as the saying goes, an unapologetic progressive, but one with more and more for which to apologize.