In many countries, constitutional amendments require the direct approval of voters, but the consequences of fundamental changes to the powers and operations of the state are difficult to anticipate. The referendums literature suggests that citizens weigh their prior beliefs about the merits of proposals against the heuristic provided by the partisanship of the proposer, but the relative salience of these factors across constitutional issue areas remains underexplored. This paper examines the determinants of citizen preferences on 12 diverse constitutional issues, based on a novel survey experiment in Japan. We show that support for amendments is greater when its proposer is described as non-partisan. However, constitutional ideology moderates this effect. Those who prefer idealistic constitutions that elevate national traditions tend to value proposals that expand government powers, compared to those who prefer pragmatic constitutions that constrain government authority. These results highlight the significance of constitutional beliefs that are independent of partisanship.