This dissertation seeks to answer the question: why do Democratic and Republican identifiers differ in their voting behavior, activism, and contributions? Republican identifiers are more likely than Democratic identifiers to turn out to vote and to vote for their chosen party rather than defect. Republicans are more likely to become activists on behalf of their party. Republicans also are more likely than Democrats to make financial contributions to their party. These partisan differences in political behavior and party loyalty persist even after controlling for the most obvious explanation: that demographic and socioeconomic differences in the type of people who identify with each party explain these observed behavioral differences. We lack theoretical explanations for why Republican mass identifiers and activists are more likely to engage in these behaviors than their Democratic counterparts. Miller and Shanks (1996, 165) argue that an 'explanation for Democratic variability and Republican constancy should rank high on electoral researchers' agenda for future research.' I argue that these partisan differences in political behavior are a product of differences in how Democratic and Republican identifiers understand and relate to their political identities. Democratic identifiers are less likely than Republican identifiers to engage in partisan political behavior because they either are more likely than Republicans to experience an internal conflict between their political identities or because they react differently than Republicans to internal conflict. Specifically, if Democratic identifiers respond more negatively to internal conflict, this also would lead them to be less likely than Republicans to engage in these key behaviors. Contrary to the conventional wisdom, I find that Republicans are not inherently more loyal than Democrats because they consistently choose to blindly follow their party. Democrats turn out less, defect more, and support their party as activists and donors less than Republicans because they are more likely to experience an internal conflict between their partisan and other group identities. Republicans appear more loyal by engaging in partisan political behavior because, quite frankly, they do not have to make the choice of group over party as often as Democrats.