How does living in a battleground state during a presidential election affect an individual's political engagement? We utilize a unique collection of 113 million Facebook status updates to compare users' political discussion during the 2008 election. Battleground state users are significantly more likely to discuss politics in the campaign season than are users in uncompetitive blackout states. Posting a political status updatea form of day-to-day engagement with politicsmediates approximate to 20 percent of the relationship between exposure to political competition and self-reported voter turnout. This paper is among the first to use a massive quantity of social media data to explain the microfoundations of how people think, feel, and act on a daily basis in response to their political environment.