Abstract: In this article, I bring two key issues in constitutional studies--institutional regime type and electoral system choice--in conversation with each other, and examine their interaction through a normative framework concerning the role that constitutions ought to play in shaping their party systems. The main goal is to offer a theoretical defense (ceteris paribus) of moderated parliamentarism--as superior to its alternatives such as presidentialism, semi-presidentialism, and other forms of parliamentarism. Moderated parliamentarism entails a strong bicameral legislature in which the two chambers are symmetric (i.e. they have equal legislative powers) and incongruent (i.e. they are likely to have different partisan compositions). It has a centrist chamber whose main function is to supply confidence to the government, and a diversified chamber whose main function is to check this government. The confidence and opposition chamber is elected on a moderated majoritarian electoral system (such as approval vote or ranked-choice/preferential vote system, but not first-past-the-post); the diversified chamber--a fully independent checking and appointing chamber--is constituted on a proportional representation model (moderated by a reasonably high threshold requirement for translating votes into seats). The confidence and opposition chamber is elected wholesale for shorter terms. It alone has the power to appoint and fire a unified political executive headed by a prime minister. The checking and appointing chamber is independent of the confidence and opposition chamber as well as of the political executive; its members have longer and staggered terms. Moderated parliamentarism combines the benefits of different regime types and electoral systems in a way that optimizes four key constitutional principles in relation to political parties: it protects the purposive autonomy of parties and enables their ability to keep the four democratic costs low; it serves the party system optimality principle by making it more likely that every salient voter type will have a party to represent it, but also distinguishes between governance parties (which are likely to dominate the confidence and opposition chamber) and influence parties (which will have a space in the checking and appointing chamber); it aids the party-state separation principle by giving significant (and over-weighted) checking powers to smaller parties in the checking chamber; and it promotes the anti-faction principle by distinguishing between smaller influence parties that are polarizing factions from those that are not factional (and punishing the latter a lot less severely than the former). The traditional debates between presidentialism and parliamentarism, and between majoritarian and proportional electoral systems have endured for as long as they have because each system brings something attractive to the table. Moderated parliamentarism seeks to combine the most attractive elements of each--checks and balance from presidentialism, anti-factionalism of majoritarian electoral systems, and political pluralism of proportional representation systems. Because these virtues are in tension, no system can maximize each of them without incurring a cost for another. Moderated parliamentarism is one way to optimize the virtues of each system and yet yield a stable and effective regime type.