A consensus exists among political scientists and the media that a two party system has become firmly established in Japan, and power alternation following the 2009 House of Representatives election seemed to confirm this trend. In contrast, in this study I draw on both election and public opinion data to illustrate that Japan's two-party system rests on fragile micro foundations, particularly since the change of government, and may therefore not constitute a stable equilibrium. Specifically, the two-party system is sustained by the logic of competition in single member districts, but lacks strong backing among a large proportion of the public. Evidence in support of this argument include 1) decreasing vote shares for both the Liberal Democratic Party and the Democratic Party of Japan in the 2010 House of Councillors election, and 2) a weakening relationship between support for these two parties in monthly polls, in contrast to previous years when LDP and DPJ ratings mirrored one another. I discuss some potential implications of these developments, such as the growing prominence of regional parties and prospects for electoral rule changes.
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