As scholars explore opportunities for democratic renewal, the potential of ballot structures to improve the quality of representation has been largely neglected. We argue that expressive ballots can improve the congruence of political preferences between voters and their vote choice and, subsequently, decrease parliamentary polarization. Recognizing that voters’ political preferences are more complex than a dichotomous party-vote allows, we propose the ‘assembly ballot’, which allows voters to choose their ‘ideal parliament’ by distributing 150 parliamentary seats across all participating parties. To assess the consequences of the assembly ballot for ideological congruence and parliamentary composition, we conducted a survey experiment with over 16,000 respondents around the 2017 Dutch parliamentary elections in which respondents cast a vote in a mock-election using the assembly ballot or a closed-list PR ballot. Results show that ideological congruence is, on average, significantly higher for voters voting with the assembly ballot for both the left–right dimension and the cultural dimension, while also producing a more centripetal, less polarized parliament.