Previously, the 3rd section of Article I of the Constitution read:
"The Senate of the United States shall be composed of two Senators from each State, chosen by the Legislature thereof, for six Years;"
Notice the difference? I highlighted it for you.
After this Amendment passed in 1913, the Federal power grab began in earnest. The states themselves no longer had any way to control their destiny through the indirect election of Senators. Senators back then would not have been as easily swayed by special interests -- given that those interests don't directly get to donate money to a campaign.
Not coincidentially, the Federal Income Tax began in 1913 with the 16th Amendment. After that, our money has gone towards supporting the Federal Government, not the states. I've long contended that our taxes should be flipped in percentages (i.e. 30-odd percent to the state, single digits to the Federal Government). That won't ever happen as long as special interests, and not the legislature of the States, can control who gets seated in the senate.