<p>This article introduces the special issue on the radicalization of the mainstream. It investigates the processes through which the far right has shifted from a marginal force to a normalized actor within contemporary democracies. It introduces the concept of the <i>radicalized mainstream</i> to capture not only the diffusion of radical ideas into public arenas, but also their internalization and amplification by mainstream actors, institutions, and discourses. The present article provides the theoretical framework with which contributions examine these dynamics across political, institutional, cultural, and communicative domains, with particular focus on the role of digital media, crises of representation, and growing polarization. Through empirical and comparative case studies in diverse national contexts, the issue demonstrates how far-right ideologies reshape the boundaries of what is considered legitimate and democratic from within. By advancing this perspective, the special issue provides new analytical tools to understand the internal transformation of democracies and the challenges posed to pluralism, participation, and rights.</p>