Thousands of Small, Constant Rallies: A Large-Scale Analysis of Partisan WhatsApp Groups

There is growing concern about the use of social platforms to push political narratives during elections. One very recent case is Brazil's, where WhatsApp is now widely perceived as a key enabler of the far-right's rise to power. In this paper, we perform a large-scale analysis of partisan WhatsApp groups to shed light on how both right-wingers and left-wingers used the platform in the 2018 Brazilian presidential election. Across its two rounds, we collected +2.8M messages from +45k users in 232 public groups (175 right-wing vs. 57 left-wing). After describing how we obtained a sample that is many times larger than previous works, we contrast right-wingers and left-wingers on their social network metrics, regional distribution of users, content-sharing habits, and most characteristic news sources.