Despite having a powerful influence on the historiography of radicalism and nineteenth-century politics for the past several decades, the language of the constitution has not recently received scholarly attention. In Chartist and radical historiography, the constitution is usually treated as a narrative of national political development. This article extends the horizons of Chartist constitutionalism by exploring its similarities with American constitutionalism. By doing so, it also opens up questions regarding the ideas of the movement. Like the Americans sixty years before, the Chartists were confronted by a parliament that they believed had superseded its constitutional authority. This perception was informed by a belief that the constitution rested on the authority of the fixed principles of fundamental law, which they argued placed limits beyond which Parliament had no power to reach. As a result, the Chartists imagined that the British constitution functioned like a written constitution. To support this claim, they drew on a sophisticated interpretation of English law that argued that the common law was closely related to natural law.