Despite ample evidence of preelection volatility in vote intentions in new democracies, scholars of comparative politics remain skeptical that campaigns affect election outcomes. Research on the United States provides a theoretical rationale for campaign effects, but shows little of it in practice in presidential elections because candidates’ media investments are about equal and voters’ accumulated political knowledge and partisan attachments make them resistant to persuasive messages. I vary these parameters by examining a new democracy where voters’ weaker partisan attachments and lower levels of political information magnify the effects of candidates’ asymmetric media investments to create large persuasion effects. The findings have implications for the generalizability of campaign effects theory to new democracies, the development of mass partisanship, candidate advertising strategies, and the specific outcome of Mexico's hotly contested 2006 presidential election. Data come primarily from the Mexico 2006 Panel Study.