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 update—a form of day-to-day engagement with politics—mediates ∼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.