This case study examines an online deliberation experiment in which a group of supporters of a large political party were invited to propose ways to reform a national electoral law. Researchers compared a traditional comment forum with the Deliberatorium, an online collaborative platform where users build “argument maps” to capture the various proposals and their associated arguments for and against. The aim of the study was to assess the capability of this tool to support large-scale deliberation in a real-world case, comparing the argument-map approach to a traditional discussion forum. By comparing users’ experience across several metrics related to usability, activity levels, and quality of collaboration, we found that while the argument-map platform was perceived as less intuitive and fluid, users nevertheless maintained their engagement at a similar rate to the forum condition and ended up producing more interactions, fewer self-referential arguments, and a more respectful tone.