The present study argues that political communication on social media is mediated by a platform's digital architecture -the technical protocols that enable, constrain, and shape user behavior in a virtual space. A framework for understanding digital architectures is introduced, and four platforms (Facebook, Twitter, Instagram, and Snapchat) are compared along the typology. Using the 2016 U.S. elections as a case, interviews with three Republican digital strategists are complimented with social media data to qualify the study's theoretical claim that a platform's network structure, functionality, algorithmic filtering, and datafication model affect political campaign strategy on social media.The structural design of an environment -its architecture -intimately affects human behavior. This interplay between structure and agency is not limited to physical environs; it also applies to how users interact with, and within, online spaces. Scholars have argued previously that a digital platform's architecture can influence, for example: the norms of interaction among users (Papacharissi, 2009), the deliberative quality of their communication (Wright & Street, 2007), or their likelihood to enact democratic ideals (Freelon, 2015). 2