Public debt has accumulated rapidly in OECD countries since 1970. We assess the political explanations for debt accumulation, paying particular attention to the US and Japan. The key finding is that political competition explains variation in debt accumulation. Political competition encourages fiscal prudence. Hence, non-competitive countries will build up debt. Thirty years of OECD experience provides strong empirical support for this conjecture, and suggests that high debt levels in Japan are more due to political than economic factors.