The voluminous literature on polarization and sorting among members of the mass public in the U.S. raises questions about why individuals choose to identify with the Republican or Democratic parties. What do individuals who identify with the Democratic (Republican) party say they like about that party, and what they dislike about the Republican (Democratic) party? How, if at all, have these explanations evolved over recent decades? This paper uses large-scale text analysis of open-ended survey responses from 1984 to 2016 to answer this question.