Congress is constitutionally bound to implement treaties. Empirically, they’re reluctant to defy the President and unwilling to make the U.S. seem uncredible.

When a treaty requires domestic legislation or an appropriation of funds to carry out United States obligations, only the Congress can supply them. As Missouri v. Holland confirmed, Congress has the power to do what is `necessary and proper' to implement a treaty even if its action might not have been within other Congressional powers.111 But as early as the Jay Treaty, debate flared as to whether Congress is constitutionally and morally obligated to implement treaties.112 In a draft for Washington's message to the House of Representatives, Hamilton wrote:

the House of Representatives have no moral power to refuse the execution of a treaty which is not contrary to the Constitution, because it pledges the public faith; and have no legal power to refuse its execution because it is a law—until at least it ceases to be a law by a regular act of revocation of the competent authority. 113

In the House, on the other hand, Representatives, including James Madison, saw in this division of power an element of checks and balances and insisted on the right (if not the duty) of Congress to determine independently the desirability of appropriations or legislation.114 The House resolved:

when a Treaty stipulates regulations on any of the subjects submitted by the Constitution to the power of Congress, it must depend, for its execution, as to such stipulations, on a law or laws to be passed by Congress. And it is the Constitutional right and duty of the House of Representatives, in all such cases, to deliberate on the expediency or inexpediency of carrying such Treaty into effect, and to determine and act thereon, as, in their judgment, may be most conducive to the public good.115

Like some other constitutional debates, this one, too, has not been authoritatively resolved in principle.116 Hamilton's position would obviously make for a more efficient system, since otherwise treatymakers might find that they had negotiated treaties but could not assure that the United States would carry them out; effectively, Presidents would have to consult the House of Representatives (as well as the Senate) and perhaps obtain its formal consent before making the treaty. Separation and checks and balances, on the other hand, were dear to the Constitutional Fathers, and many of them were still on the scene when implementation of the Jay Treaty passed the House only barely and after bitter debate.

Since then, members of Congress have sometimes asserted the power—and the right—not to carry out international obligations, but Congresses have in fact not failed to do so though sometimes they are slow to do so (as, for example, in the 1990s, the treaty obligations to pay U.S. dues to the United Nations). In general, Congress has responded to a sense of duty to carry out what the treatymakers promised, to a reluctance to defy and confront the President (especially after he can no longer retreat), to an unwillingness to make the U.S. system appear undependable, even ludicrous. But the independence of the legislative process (subject only to the Presidential veto as provided in the Constitution) has given Congress opportunities to interpret the need for implementation and to shape and limit it in important details; Congress has not always given the President exactly the laws he asked for or as much money as he said a treaty required.*