On why the "General Welfare" clause does not confer a power to legislate:
"It would reduce the whole instrument to a single phrase, that of instituting a Congress with power to do whatever would be for the good of the United States; and as they would be the sole judges of the good or evil, it would be also a power to do whatever evil they please. Certainly no such universal power was meant to be given them." -- Thomas Jefferson
"It has been urged and echoed that the power 'to lay and collect taxes, duties, imposts, and excises, to pay the debts, and provide for the common defense and general welfare of the United States,' amounts to an unlimited commission to exercise every power which may be alleged to be necessary for the common defense or general welfare. ... Had no other enumeration or definition of the powers of the Congress been found in the Constitution than the general expressions just cited, the authors of the objection might have had some color for it; though it would be difficult to find a reason for so awkward a form of describing an authority to legislate in all possible cases." -- James Madison, in Federalist #41.