<p>This dissertation proposes that party identification, as a social identity, fundamentally alters individual processing of and reactions to political information and events. I present a <italic>party identity linkage theory</italic> in which I argue party identity can lead to heightened, specific emotional responses to threatening political competition and biased, polarized perceptions of politicalized objects if the link between self and party is sufficiently strong. Because people are strongly motivated to protect the positive perceptions they have of themselves, they should be motivated to maintain and protect their positive perceptions of groups that are linked to their self-concept through social identities. Furthermore, because people tend to engage in self-serving biases that result in a degree of positive illusions about themselves, especially when the positive self-view is threatened, evaluations of closely linked groups should also be subject to a degree of positive bias, especially when the positive image of the group is threatened. Drawing on both experimental and survey data, I provide evidence that strong partisans are fundamentally different from weak partisans and independents in the degree a party is included in their self-concepts, in their responses to candidates' changed party status, and in their responses to threatening inter-party competition.</p> / Dissertation