Art. I, Section 8

The Congress shall have Power... to pay the Debts and provide for
 the common Defence and general Welfare of the United States

The Supreme Court's Test for Exercise of the Spending Power (South Dakota v Dole)

1.  Is the spending "in pursuit of the general welfare"?

2.  If Congress has conditioned the States' receipt of federal funds, are the conditions unambiguous?

3.  Are any conditions related to a federal interest in a national program or project?

4.  Do any of the conditions induce the States to take actions that themselves would be unconstitutional?

5.  Are the financial inducements to States so great as to be "financial compulsion" and not just a "temptation"?