This dissertation aims to investigate the bases of partisan differentiation and degree of polarization since Taiwan's 2000 presidential election. By employing American concepts and theories of partisan polarization, I analyze Taiwan's party politics at both the elite and mass levels. At the elite level, I examine whether inter‐party antagonism has become more intense in Taiwan's legislature and what types of issues contribute most to party conflict since 2000. At the mass level, I examine public perceptions of the parties, analyze whether any political issues divide the Taiwanese public along partisan lines, and explore the social and demographic bases of partisan divisions. The findings suggest that political elites became polarized along partisan lines after 2000, as observed in roll‐call voting behavior in the Legislative Yuan. This resulted from the formation of a divided government and the confrontation of two party coalitions after the 2000 presidential election. Furthermore, this polarization is mainly due to the opposite positions of the two party coalitions on the issue of the relationship with China. The pan‐blue party coalition favors reunification and closer interaction with China, whereas its counterpart, the pan‐green party coalition, favors Taiwanese independence and limited interaction with China. The issues of social reform vs. stability, social welfare vs. lower taxes, and environmental protection vs. economic development are less polarizing and less consistently divisive than the issue of Taiwan's relations with China. Partisan polarization is less evident among ordinary citizens than among political elites. The only issue dividing Taiwanese significantly is the China relationship issue (independence or unification with China). In addition, demographic factors may lead to partisan division among citizens. Nevertheless, this polarization is more moderate than that of political elites because the number of partisan independents is high and has not decreased significantly. In short, partisan polarization in Taiwan is not as intense as some political scientists claim. Taiwan's partisan polarization at the mass level is closer to the concept of "sorting", referring to the process of people gradually affiliating with the party that best reflects their policy preferences, even if those preferences are more moderate than extreme.

Since the mid-1970s, partisan polarization has been increasing in Congress and the Presidency, and, although most voters lack a stable, consistent ideology, non-ideological forms of partisan polarization have emerged in the mass public in recent decades. Moreover, ideological polarization among elites is highly asymmetrical, with increased Republican conservatism accounting for most of the increased ideological distance between the parties. Here, I develop a racial-threat backlash theory and argue that increased rates of immigration are associated with increased asymmetrical ideological polarization among elites and in the mass public. Tests of this theory on voters, the mass public in the states, state legislators, and Senators provide support for my theory. In addition, when accounting for the effects of immigration, I do not find support for the alternative explanation that increased income inequality leads to increased asymmetrical partisan polarization.

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