Successful coordination around a Duvergerian equilibrium requires accurate and consistent information about parties’ expected electoral support. In practice, such information is often unreliable and rarely available at the local level, thus hindering voters’ coordination. In this paper we leverage Argentina’s Open, Mandatory and Simultaneous Primary Elections as a large-scale survey of voter preferences. Using data from 135 municipalities in the province of Buenos Aires (2011-2023), we show that a narrower margin between the top-two placed parties in the primary increases both turnout and the proportion of positive votes in the general election, while decreasing electoral fragmentation. In line with Cox’s (1997) expectations, we further show that the second-placed party in the primary is substantially more likely to win the election than the third-placed one. Consistent with theoretical predictions, these effects are more pronounced (a) in concurrent elections; (b) in smaller municipalities; and (c) when the second-placed party is closer to the first-placed one.