The Compact Clause of the U.S. Constitution states, “No State shall, without the consent of Congress, . . . enter into any agreement or Compact with another State . . . .” U.S. CONST. art. I, § 10, cl. 3. Though a strict reading of the Compact Clause might appear to require congressional consent for every Compact, the Supreme Court has determined that “any agreement or Compact” does not mean every agreement or Compact. The Compact Clause triggers only by those agreements that would alter the balance of political power between the states and federal government, intrude on a power reserved to Congress, or alter the balance of political power between the Compacting states and non-Compacting states. Virginia v. Tennessee, 148 U.S. 503, 519 (1893) (agreements “which may tend to increase and build up the political influence of the contracting States, so as to encroach upon or impair the supremacy of the United States . . . .”); U.S. Steel Corp. v. Multistate Tax Comm’n, 434 U.S. 452, 495–96 (1978) (non-Compact states placed at competitive disadvantage by the Multistate Tax Compact); Ne. Bancorp v. Bd. of Governors of Fed. Reserve Sys., 472 U.S. 159, 176 (1985) (statute in question neither enhances the political power of the New England states at the expense of other states or impacts the federal structure of government).

Where an interstate agreement facilitates only what states could accomplish unilaterally, the Compact does not intrude on federal interests requiring congressional consent. See U.S. Steel Corp., 434 U.S. at 472–78. The lack of requisite congressional consent, however, does not affect the contractual nature of the agreement between states.

Congress does not pass upon a Compact in the same manner as a court decides a question of law. Congressional consent is an act of political judgment about the Compact’s potential impact on national interests, not a legal judgment as to the correctness of the form and substance of the agreement. See Detroit Int’l Bridge Co. v. Gov’t of Canada, 883 F.3d 895, 899 (D.C. Cir. 2018). Implied consent may exist when actions by the states and federal government indicate that Congress has granted its consent even in the absence of a specific legislative act. See Virginia v. Tennessee, 148 U.S. at 521–22.

Alternatively, Congress may attach conditions to its consent. Conditions can be proscriptive involving the duration of the agreement. Other congressional conditions may be compulsory, requiring member states to act in a certain manner before activation of the Compact. On the other hand, conditions authored by Congress can be substantive, altering the purposes or procedures mandated by a Compact. The only limitation imposed on congressional conditions is that they must be Constitutional. New York v. United States, 505 U.S. 144 (1992). Courts deem that states that adopt an interstate Compact to which Congress attaches conditions have accepted those conditions as a part of the Compact. Petty v. Tennessee-Missouri Bridge Comm’n, 359 U.S. 275, 277-78 (1959) (mandated provisions regarding suability of bridge commission as binding on states because Congress possessed the authority to impose conditions as part of its consent, and the states accepted those conditions by enacting the Compact).

When states amend a Compact with consent, Congress must assent to the amendment. However, there is no requirement for additional consent if the amendment is consistent with Congress’ existing authority. See, e.g., Joint resolution granting consent to amendments to the Compact between Missouri and Illinois, Pub. L. No. 112-71, 125 Stat. 775 (2011); Int’l Union of Operating Eng’rs, Local 542 v. Delaware River Joint Toll Bridge Comm’n, 311 F.3d 273, 280 n.7 (3d Cir. 2002) (where a Compact contains no provision for amendment, congressional consent to any modification would be necessary).