In the first decades of the nineteenth century, political thinkers in the United States worked within a theoretical consensus in favour of popular sovereignty. Jacksonian Democrats put this theory into practice with majoritarian democracy and with constitutional conventions through which ‘the people’ directly exercised their power. Conservative sceptics of democracy, however, were not as enthusiastic about a regular role for the people in routine governance and were especially opposed to their ability to reformulate constitutional order at will through conventions. Actual popular sovereignty, particularly when used to revise constitutions, struck conservatives as a dangerous return to the state of nature. Conservatives expressed anxiety over popular sovereignty by likening the majority exercising sovereignty in constitutional conventions to the populist demagogue, who would seize upon the people’s sovereignty to upend constitutional order. The tyranny of the majority and the usurpation of the demagogue acting in the people’s name seemed one and the same, because both the sovereign people and the populist usurper ignored the rule of law and abided by the laws of nature. For conservative thinkers, the American republic was uniquely susceptible to populist usurpation because a radical promise of popular sovereignty rested at its foundation. Examining the conflicting attitudes towards popular politics held by Jacksonian Democrats and their conservative critics, this chapter explains how political thinkers in the early American republic understood the connection between popular sovereignty and populism, and how, for conservatives, the uniquely American theory of popular sovereignty slipped all too easily into a distinctly American tyranny.