In a column titled "How Might Makes Right [lewrockwell.com] ," Mr. Sobran writes:
The Constitution sounds great on paper. But how is the Federal Government to be prevented from exceeding its allotted powers? Originally there were three safeguards...Second, the Senate of the United States represented the states, and would oppose any usurpation of the rights reserved to the states and denied to the Federal Government. But the Seventeenth Amendment virtually abolished the Senate by requiring the popular election of senators, ending their selection by the state legislatures. By being democratized, the Senate became a redundant institution, with no special constitutional function.
-PJ
I bet one of the other two safeguards was the prohibition on direct taxation, with the exception of taxes used to pay down debts.