Money cannot be applied to the General Welfare, otherwise than by an application of it to some particular measure conducive to the General Welfare. Whenever, therefore, money has been raised by the General Authority, and is to be applied to a particular measure, a question arises whether the particular measure be within the enumerated authorities vested in Congress. If it be, the money requisite for it may be applied to it; if it be not, no such application can be made.
-James Madison
[O]ur tenet ever was, and, indeed, it is almost the only landmark which now divides the federalists from the republicans, that Congress has not unlimited powers to provide for the general welfare, but were to those specifically enumerated; and that, as it was never meant they should raise money for purposes which the enumeration did not place under their action; consequently, that the specification of powers is a limitation of the purposes for which they may raise money.
-Thomas Jefferson
If Congress can do whatever in their discretion can be done by money, and will promote the General Welfare, the Government is no longer a limited one, possessing enumerated powers, but an indefinite one, subject to particular exceptions.
-James Madison