The day after President Trump announced the United States’ withdrawal from the Paris climate agreement, the governors of New York, California, and Washington voiced their dissent by forming the “United States Climate Alliance.” This coalition asks individual states to remain committed to the main tenets of the Paris accord, including reducing greenhouse gas emissions by 26-28%. Although the actual terms of the alliance remain undefined, governors are already signing accompanying state executive orders that call for implementing emissions reductions. So far, the governors of Rhode Island, Connecticut, Vermont, Oregon, Hawaii, and Massachusetts have also pledged to join.

But these efforts could face constitutional hurdles. The Constitution’s oft-neglected Compact Clause provides that no state may enter an agreement or compact with another state (or a foreign power) without the consent of Congress. The Framers enacted this provision to prevent interstate collusion against the federal government or the interests of other states. In particular, the Framers wanted to ensure that only the national government spoke for the United States on matters of international affairs; states are not permitted to have their own foreign policy. In short, the Compact Clause appears to have been designed to prohibit exactly the sort of agreement that New York, California, and others now propose to enter. To the extent that the Climate Alliance proposes to adopt actual policies, as opposed to mere aspirational statements in support of reducing global carbon emissions, both the text of the Compact Clause and its placement in the Constitution—in a section of Article I limiting state power—suggest that the Alliance cannot act without congressional approval.

On the other hand, although the Constitution’s text says that all interstate agreements require congressional approval, the Supreme Court has narrowed the application of this clause to only compacts that increase state power at the expense of federal supremacy. The seminal case is Virginia v. Tennessee (1893), in which Justice Field, for a unanimous Court, declared the impracticality of requiring Congress to approve all interstate agreements. In extended dicta, the Court noted that the terms “agreement” and “compact” are comprehensive enough to apply to all forms of stipulation amongst states. But the Court was reluctant to give such a “literal” reading for fear of imposing unnecessary barriers on state cooperation, so it held instead that the consent requirement applies only to those compacts that increase the member states’ political power relative to the federal government. Agreements that do not “encroach upon” the “supremacy of the United States” do not require consent.

Subsequent cases have affirmed this functional “federal supremacy” test. In U.S. Steel Corp. v. Multistate Tax Commission (1978), the Court built on Virginia v. Tennessee and articulated three standards that allow interstate compacts to avoid otherwise constitutionally mandated congressional review. An interstate agreement does not improperly increase state power if the agreement: (1) does not authorize member states to exercise powers beyond those they already have; (2) does not delegate sovereign power to the administrative body created by the compact; and (3) allows a state to withdraw at any time.

In addition to the federal supremacy test, the Court has also limited the scope of what qualifies as a “compact,” thereby avoiding application of the Clause altogether. In Northeast Bancorp v. Board of Governors of the Federal Reserve (1985), the Court identified four “classic indicia of a compact”: (1) setting up a regulatory body; (2) conditioning action on corresponding actions of other participants; (3) restricting a participant’s ability to modify or repeal its own laws; and, (4) reciprocal constraints on each State’s regulations.

The Court’s precedents thus suggest that states have more flexibility than might otherwise appear from the Constitution’s text. By creating carefully worded agreements that call only for states to exercise their traditional functions, that do not delegate sovereign power, and that appear nonbinding, states can avoid the narrow range of circumstances that the Court has found to trigger mandatory congressional permission.

The specifics of the United States Climate Alliance are not yet known. It is unclear what, if any, legal obligations member states will incur. If the agreement calls for nothing more than the same non-binding commitments that the Paris accord presented, then it likely will not violate the Compact Clause. But if the states seek to set an example by committing themselves and their co-signers to binding emissions reductions as ostensible participation in a multinational agreement, then the Alliance could push the boundary of Compact Clause doctrine.

The Supreme Court’s Compact Clause jurisprudence is both thinly developed and severely dated. The Court hasn’t touched a Compact Clause case since 1985 and no court has struck down a compact for lack of congressional approval since 1893. Litigation over interstate compacts has been sparse, but states’ efforts to combat climate change are sure to provoke legal challenges. And those challenges could cause the current Court to revisit the doctrine altogether. The Court’s precedents can be fairly criticized for effectively inverting the text of the Compact Clause by switching the default presumption against interstate compacts to a position allowing states’ agreements largely unfettered. The modern Court may not be willing to afford the states such broad discretion, especially if it appears that some states have attempted to join an international agreement notwithstanding the President’s withdrawal, or to enact serious carbon-emission regulations with the potential to affect the national energy grid notwithstanding the federal government’s different regulatory balance.

As the Climate Alliance sets out to advocate for the role of the states in combatting climate change, it will be important to bear in mind the Constitution’s text.

Stephanie A. Maloney is an attorney at Winston & Strawn LLP in Washington, DC.