The standard explanation for the rise of medieval legislative assemblies invokes the idea that rulers lacking resources were forced to bargain with the societal actors who control them, thus conceding rights and representation. Yet, while most previous works have focused on the bargaining process between early representative assemblies and the monarch, we do not know much about how legislative actors behaved during these power voids. We contend that elites in early representative assemblies used these power struggles to signal their sovereignty over the realm. We empirically examine this logic using the activity reported in the XVII century in the Journals of the House of Commons and the House of Lords, two of the longest established representative assemblies in the world. In order to capture the power of both representative assemblies, we implement a novel strategy based on entropy of topic shares in daily records of parliamentary activity. Our findings show that elites in both representative assemblies strategically used power voids in order to signal their sovereignty preferences and eventually extract concessions from the Monarchy. Our findings have important implications for our understanding of early legislative assemblies and provide a new way of measuring institutional power through the analysis of text.