While much is known about who votes in American elections, much less is known about who donates. In this book, the authors utilize a unique and historically unprecedented data set of donors from the 2008 and 2012 presidential elections to answer longstanding questions: What is the relationship between donors and candidates? How do candidates attract and respond to contributors? How do campaign strategies reflect changing campaign finance laws and the development of the internet? With unprecedented cooperation from the Obama, McCain, and Romney campaigns, the authors investigate presidential campaign donors at all giving levels to produce the most systematic and complete analysis of donors to presidential nominees to date. As elections are decided increasingly by donors' dollars, Who Donates? provides relevant research on the broader trends in partisan polarization and, more generally, on how campaigns can engage more citizens in political participation.