Citizens unequally participate in referendums, and this may systematically bias policy in favor of those who vote. Some view compulsory voting as an important tool to alleviate this problem, whereas others worry about its detrimental effects on the legitimacy and quality of democratic decision making. So far, however, we lack systematic knowledge about the causal effect of compulsory voting on public policy. We argue that sanctioned compulsory voting mobilizes citizens at the bottom of the income distribution and that this translates into an increase in support for leftist policies. We empirically explore the effects of a sanctioned compulsory voting law on direct-democratic decision making in Switzerland. We find that compulsory voting significantly increases electoral support for leftist policy positions in referendums by up to 20 percentage points. We discuss the implications of these results for our understanding of the policy consequences of electoral institutions.