Recent events have shown how uncivil communication by political elites on social media can spread through networked publics and contribute to real-world disruption. To explain the upstream dynamics of these processes, this book introduces the Spiral of Political Incivility on Social Media (SPIN), a model that conceptualizes incivility as a dynamic, three-stage process: the production of norm violations by political actors, their perception shaped by platform affordances and national contexts, and user reactions that may reinforce or dampen the original cues. Drawing on a most-different-systems design, the study combines computational text analysis of 37,152 campaign posts across Facebook, Twitter, and Telegram in Brazil, Spain, and the United States, representative surveys (N = 2,337), and an analysis of 13.6 million user comments. The findings reveal three ideal-typical trajectories—reinforcing, reactive, and dampened spirals—demonstrating that political incivility online is not random noise but a context-dependent process shaped by the interaction of actors, platforms, and users. By identifying where these spirals accelerate or stall, the study highlights key leverage points for mitigating the diffusion of incivility in digital democracies.