Political parties’ behavior in coalition formation is commonly explained by their policy-, vote-, and office-seeking incentives. From these perspectives, the 20-year partnership of Japan's ruling conservative Liberal Democratic Party (LDP) and its pacifistic Komeito junior coalition partner is an anomalous case. The longevity, closeness, and nature of their unlikely partnership challenges core assumptions in existing theories of coalition politics. LDP–Komeito cooperation has sustained for two decades despite vastly different support bases and ideological differences on fundamental policy issues. LDP leaders also show no signs of abandoning the much smaller Komeito despite enjoying a single-party majority. We argue that the remarkable durability of this puzzling partnership results primarily from the two parties’ electoral incentives and what has effectively become codependence under Japan's mixed electoral system. Our analysis also demonstrates that being in a coalition can induce significant policy compromises, even from a much larger senior partner. Beyond theoretical implications, these phenomena yield important real-world consequences for Japanese politics: especially, a far less dominant LDP than the party's Diet seat total suggests, and Komeito's remarkable ability to punch significantly above its weight and constrain its far larger senior partner, even on the latter's major national security policy priorities.

The dominance of Japan's Liberal Democratic Party (LDP) was long buttressed by the existence of a strong political support base in the rural areas led by local politicians who worked on behalf of national LDP politicians seeking reelection. In recent years, municipal mergers have drastically weakened the LDP's support base by reducing the number of local politicians and redrawing electoral district boundaries. Surprisingly, the main opposition party, the Democratic Party of Japan (DPJ), could not take full advantage of these new institutional arrangements. Instead, local politicians have become more independent of both major parties. As a result, at a time of increasing numbers of floating voters, neither of Japan's two major parties has a reliable local base across the country. To succeed, both parties must pay attention to the changing needs of the increasingly independent—and very often still rural—localities.

Particularistic spending has played a storied role in Japanese politics, but during the last decade of LDP rule, expenditures on roads, bridges, agricultural projects, and the like steadily lost ground to more programmatic outlays on social welfare, science and technology, and public order (but not defense or foreign aid). Prime Minister Koizumi played an important role in this shift, but the trends preceded him and continued under his much weaker successors. The end of the Cold War, increasing foreign investment, and the weakness of the domestic economy probably played mostly minor roles in the decline of particularism. The aging of Japanese society, not least in rural areas, created direct pressure for programmatic spending, while partisan upheaval, the growing share of floating voters, and reforms to the electoral and administrative systems created both an incentive and a greater capacity to redirect attention to the concerns of median voters.

By examining party-switching decisions among members of Japan's Liberal Democratic Party (LDP), this article shows how distributive policy programs exclusively available to the governing party attract incumbents to the party in power. In a stable electoral environment where the government party is likely to stay in power, legislators elected from infrastructure-poor constituencies are effectively tied to the party. However, when the party's electoral prospects are uncertain, legislators behave more sincerely and switch parties to match their policy preferences. It is also found that defectors elected from infrastructure-poor constituencies tended to return to the LDP once the party installed a stable surplus coalition.