Of course, all of this is from a purely theoretical and academic point of view. Naturally, adopting Madison's interpretation would have catastrophic consequences.
There were really three views on the spending power:
1. Madison would have limited the spending power to the objects explicitly enumerated in the Constitution.
2. Hamilton treated the words "General Welfare" as a policy question, and thus would have allowed Congress to spend money on anything it wanted.
3. Monroe took a middle ground position, eventually embraced even by Hamilton himself, which is very hard to define.
I agree that Madison's position is the correct one. But I think the Court can place real limits on the spending power without totally disavowing its more recent case law by adopting the middle ground position of Monroe.
But how should we define it? My idea flows directly from the Sixth Virginia Resolution of 1787, which states:
"[T]hat the National Legislature ought to possess the Legislative Rights vested in Congress by the Confederation; and moreover, to legislate in all cases for the general interests of the union, and also in those cases to which the States are separately incompetent, or in which the harmony of the United States may be interrupted by the exercise of individual Legislation."
There is no doubt that this resolution reflects much of the spirit behind the Constitution. It was sent to the Committee on Detail, where it was then transformed into the Enumeration of Powers in Article I, Section 8 -- including the spending power.
The words "General" and "Common" in the spending power seem to suggest that it should be limited to matters "to which the States are separately incompetent." I think that is the best test that could be formed.
I quote the resolution not because of any importance it has on its own, but for those specific words, which we know at least reflect the constitutionally legitimate principle of federalism, even when taken as a rule rather than a generic request.