To mitigate the economic consequences of the COVID-19 pandemic, governments have spent and borrowed on a scale not seen since the Second World War. Government debt is thus likely to become a hot political issue again, yet we still know little about how people reason about debt sustainability. How do concerns over high debt levels compare with concerns over other salient issues such as climate change or illegal migration? How malleable are these concerns and with what consequences for people’s policy preferences and their expectations for future fiscal adjustment? We use observational and experimental data collected in Great Britain to answer these questions. We purposely designed instructional videos that summarize two different sides of the debate over debt sustainability: one video emphasizes the unprecedented levels of government debt, while the other video emphasizes the low interest rates that keep governments’ borrowing low. We randomly expose respondents to these videos and use a within-person design to examine whether watching the video changes expressed concerns about debt. Moreover, we use a between-person design to examine whether the videos affect respondents’ fiscal policy preferences and priorities as well as their expectations about future growth, inflation, and the likelihood of fiscal adjustment. This allows us to examine whether respondents have a widely shared mental model regarding the consequences of high debt. In the absence of such a mental model, politicians face fewer constraints from public opinion than commonly suggested.