What would be the difference, in terms of U.S. federal and international enforceability, of an outcome that adopts treaty text that is later approved by a congressional-executive agreement, compared to the adoption of that same text as a COP decision, combined with a U.S. unilateral declaration of its intent to be bound by that decision?
A treaty is more likely to be enforceable. A unilateral statement can be binding as a matter of international law, but its enforceability in U.S. domestic law is much more doubtful and limited.
Under U.S. law, a treaty is on a par with federal statutes but the treaty’s provisions will only be enforceable over prior federal laws to the degree the treaty provisions are considered “self-executing”, i.e enforceable without the need for implementing laws or regulations. Whether a treaty will be treated as self-executing in US domestic law can be influenced by provisions of the treaty that so state, but also will depend on the statements of the President in submitting the agreement for advice and consent of the Senate, and the statements of the Senate. In practice, economic regulatory treaties have most often been treated as non-self-executing, at least with regard to some of their provisions, with the result that implementation has required not only Senate advice and consent to ratification, but implementing legislation.
It is well established as a matter of US law that the United States can also enter into international agreements without going through the treaty process. Some have argued that there are certain types of agreement that “must” receive Senate advice and consent as “treaties”, but the courts have never so ruled nor are we aware of any instance in which an agreement submitted for approval by legislation was rejected on grounds it should have been submitted for Senate advice and consent. Tradition does have a role, however, and some will argue that multilateral environmental agreements should be submitted as treaties in the United States, as in the case of agreements such as the Montreal Protocol.
In some cases, the President has entered into what the United States calls “executive agreements”, without obtaining either Congressional approval of the agreement or new legislation, if the President considers that he has sufficient existing constitutional or legislative authority to carry out US obligations without breaching any existing federal law. Even in such cases, the President will sometimes decide to seek legislative approval of the agreement (by joint resolution or a new statute), in order to give the agreement greater stature and to make less likely that Congress will in the future pass laws that are inconsistent with the agreement. An executive agreement that is not Congressionally approved or implemented cannot override inconsistent federal law
In the case of an agreement that would require a change in existing federal law, the President can choose simply to seek new legislation making the necessary changes, without obtaining Congressional approval of the agreement itself. However, in the interest of obtaining some congressional “buy-in” that will help prevent future unravelling of the agreement, the President will frequently ask Congress to enact legislation that both approves and implements the agreement. All the major trade agreements, such as the WTO and the NAFTA, were (in US terminology) executive agreements that were approved and implemented by legislation. None were done as treaties, as the United States understands that term. (Other nations, and international law generally, do not make the same vocabulary distinction, and so may use the term “treaty” for an agreement that in US law would be called an “executive agreement”).
Under U.S. law, subsequent federal legislation will supersede a prior treaty or Congressionally approved executive agreement to the extent that the subsequent legislation requires a violation of the agreement. That is so even if, in addition to Senate advice and consent to ratification or approval by joint Congressional action, Congress ha enacted implementing legislation for the agreement. International agreements do not prevail over inconsistent subsequent federal legislation. The element of Congressional approval, through the treaty process or otherwise, make subsequent inconsistent legislation politically less likely, but does not eliminate the legal possibility.
We are not aware of any instance in which US courts have found truly unilateral US declarations to be enforceable. The President can honor his unilateral declarations to the extent that the declarations are not inconsistent with federal law, but the President could not ignore any federal law on grounds that it was inconsistent with a simply unilateral declaration. Further, it is doubtful that courts would require the president to comply with a unilateral declaration even if the President had authority to do so, unless compliance was required by the terms of the federal law itself, regardless of the declaration.
In theory, to the degree a unilateral declaration were considered to create an international obligation of the United States, the courts could decide that US law should be interpreted in a way that complied with that obligation, if the law reasonably permitted such an interpretation. As a practical matter, however, this situation is unlikely to arise. If the President considers that US law can be interpreted consistent with a unilateral declaration that the President considers internationally binding, the President will do so. On the other hand, the courts are unlikely to find a unilateral declaration to be internationally binding over the objections of the executive branch.
The problem of domestic enforceability is not overcome by the nature of drafting of a unilateral declaration, since the critical point is the need for Congressional action to change existing federal law and to help ward off future inconsistent measures.