This article explores the impact of federalism on national party cohesion. Although credited with increasing economic growth and managing conflict in countries with diverse electorates, federal forms of government have also been blamed for weak party systems because national coalitions may be divided by interstate conflicts. This latter notion has been widely asserted, but there is virtually no empirical evidence of the relationship or even an effort to isolate and identify the specific features of federal systems that might weaken parties. In this article, I build and test a model of federal effects in national legislatures. I apply my framework to Brazil, whose weak party system is attributed, in part, to that country's federal form of government. I find that federalism does significantly reduce party cohesion and that this effect can be tied to multiple state‐level interests but that state‐level actors' impact on national party cohesion is surprisingly small.