Political struggles are commonly analysed as unified projects of collective action oriented towards external adversaries. This assumption obscures a central political fact: struggles themselves operate as fields of power. This article advances Internal Contestation Theory to explain how collective struggles generate internal hierarchies, exclusions, and authority long before any external victory or institutional transition occurs. Rather than treating internal conflict as fragmentation or organisational failure, the theory conceptualises struggle as a political field in which actors compete to capture legitimacy, control resources, regulate voice, and pre-authorise future authority. I argue that internal contestation unfolds through a sequenced process of legitimacy capture, gatekeeping, voice regulation, resource conversion, and anticipatory rule consolidation. These mechanisms transform collective action into structured hierarchies that shape representation, decision-making, and political outcomes both during and after periods of mobilisation. The article adopts a conceptual theory-building methodology, synthesising insights from political sociology, contentious politics, institutionalism, and legitimacy theory to develop a generalisable analytical framework that relies on no case-specific evidence. The contribution is threefold. First, it reconceptualises struggle as a field of internal power rather than moral unity. Second, it offers a portable model for analysing legitimacy capture within collective action. Third, it reframes debates on representation and democratic futures by demonstrating how internal power dynamics prefigure post-struggle governance.