I would say this is a state's rights issue, this issue has been hotly debated, and at one time even caused the United States to break into a bloody civil war.
Congress is given, in Article I, Section 8, specifically the power to regulate commerce BETWEEN states, and with foreign nations, the right to regulate commerce between states (which, IMHO a national license for tradesmen would do) has long been held as a fundamental right of each individual state (I.E. a state can tax something bought in the state, but only the federal government can tax goods moving between two states) so, if I'm reading the situation right, such a proposal would be unconstitutional.
Remember, the U.S. was not always one nation, it has been 14 several nations (Texas was independant until at asked to be annexed by the United States) and many powers of government are still reserved for the individual states.
