Political parties are institutions that are integral to the functioning of constitutional democracy and should be conceptualized as public law institutions, alongside courts, executives, and legislatures. But political parties have been under-theorized in public law theory, which as a consequence possesses relatively few intellectual resources to understand, assess and propose a response to the current state of political party systems. A public theory of political parties can be built from two sources. First, it should be rooted legal materials and institutional practices, and offer an interpretive reconstruction of them which abstracts away from the particulars of how political party systems operate, to provide a critical standard which serves as a normative guide for assessing current practice. Second, it should be based on a careful rereading of the early constitutional theorists of parliamentary democracy – including Bagehot and Dicey.