In new democracies party systems are often young, so partisan cues and roots in the electorate tend to be weak. The results, in many instances, include volatile campaigns with comparatively high degrees of short-term preference change among voters. We explore the mechanisms of voter volatility and, more broadly, the ways in which citizens learn about issues and candidates in weak-party systems. We claim that citizens in such settings rely heavily upon persuasive information gathered from their immediate social contexts. Utilizing a unique panel survey implemented during Brazil's historic 2002 presidential election, we demonstrate the importance of political discussion within social networks and neighborhood context for explaining preference change during election campaigns. We also demonstrate the concrete political consequences of social context by showing how candidate momentum runs can be driven by waves of discussion.