This Article constructs frameworks for analyzing federalism’s undertheorized horizontal dimension. Discussions of federalism generally focus on the hierarchical (or vertical) allocation of power between the national and state governments while overlooking the horizontal allocation of power among coequal states. Models of federal-state relations tend to treat the fifty states as a single aggregate unit, obscuring the fact that individual states often cannot concurrently exercise their powers without infringing the other states’ autonomy, frustrating the others’ legitimate interests, or burdening the others’ citizens. Preserving interstate harmony and protecting citizens from excessive burdens therefore requires limits on how states may wield their shared authority. Constitutional law currently addresses these limits in a piecemeal fashion through doctrines regulating such diverse subjects as personal jurisdiction, restraints on interstate commerce, choice of law, federal subject-matter jurisdiction, interstate compacts, federal common law, tax apportionment, interjurisdictional preclusion, and discrimination based on state citizenship. This Article moves beyond the piecemeal approach by identifying facets of horizontal federalism that transcend doctrinal categories. Considering these common features without the distraction of historically contingent doctrinal labels can help reconfigure jurisprudence that is often unprincipled, unsatisfying, and unstable.
The Article proceeds in four steps. First, it defines horizontal federalism, explains how horizontal and vertical federalism overlap, and explores structural features of the Constitution that complicate efforts to define limits on state authority. Second, it groups seemingly unrelated examples of state action into eight categories. This typology highlights thematic connections between forms of state action that prevailing doctrine often treats separately. Third, the Article analyzes the Constitution holistically to identify the clauses that regulate horizontal federalism and consider how these fragments fit together to resolve, deter, or mitigate problems arising from the categories of state action noted above. This approach identifies five methods that the Constitution uses to regulate interstate activity.
Finally, the Article develops a model for analyzing jurisprudence implementing the Constitution’s methods for coping with horizontal federalism. This model reveals that horizontal federalism doctrines rely on a varying combination of four concepts: capacity (the scope of a state’s authority), constraint (rights or immunities that limit state power), centralization (express or implied federal preemption or authorization of state action), and comity (the need for states to respect each other even when capacity exists free from constraint or central control). Identifying these concepts exposes at least three sources of incoherence or instability within horizontal federalism jurisprudence. First, judicial decisions are often imprecise about which concept controls, leading to a lack of fit between reasoning and outcomes. Second, the role of the four concepts can vacillate within a line of precedent over time, leading to confusion about a doctrine’s rationale and proper application. Finally, distinct lines of precedent can deploy the four concepts differently despite the lack of meaningful distinctions between the doctrines’ underlying purposes or functions. Parsing and critiquing capacity, constraint, centralization, and comity arguments can therefore affect the implementation, justification, and coordination of horizontal federalism doctrines. The model thus provides a foundation for future scholarship reevaluating vast swaths of constantly evolving law.