Article I, Section 8, Clause 18:
[The Congress shall have Power . . .] To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.
Reinforced by the necessary and proper clause, the powers
‘to lay and collect taxes, to pay the debts and provide for the common defence and general welfare of the United States,’ and ‘to borrow money on the credit of the United States and to coin money and regulate the value thereon . . . ,’ 1 have been held to give Congress virtually complete control over money and currency. A prohibitive tax on the notes of state banks, 2 the issuance of treasury notes impressed with the quality of legal tender in payment of private debts 3 and the abrogation of clauses in private contracts, which called for payment in gold coin, 4 were sustained as appropriate measures for carrying into effect some or all of the foregoing powers.