自民黨從1999年10月開始邀請公明黨進入聯合政權迄今，合作關係未曾中斷，合作的時間與次數均超過其他的合作政黨。一般觀念中的「關鍵少數」是指，國會議席是否過半數的必要少數席次。但是，本文認為，公明黨所扮演的關鍵少數的實質意義，不在於「選舉後」國會席次的數目，而是在於「選舉前」選民對執政聯盟的支持，換言之，公明黨支持者對自民黨候選人，特別是眾議院小選舉區候選人的支持，是自民黨繼續成為最大政黨的主要力量，也是維繫聯合政權的主要裝置。 本文首先針對聯合政權組成動機的既有觀點提出檢討，繼而論述自民黨與公明黨的困境，以說明自公兩黨的合作動機，最後利用眾參二院的選舉資料說明公明黨的合作效果。

Ever since the Liberal-Democratic Party (LDP) invited the New Komeito to form a coalition government, which began governing in October 1999, the partnership between the two parties has yet to break down. The duration and importance of this partnership is remarkable in Japanese government. Generally speaking, a party represents a “critical minority” if it controls the bloc of seats that are decisive in forming a government. However, this paper points out that the New Komeito's role is more critical in pre-election support of the ruling coalition, rather than in post-election power in the Diet. In other words, the driving force behind the partnership and one reason for the LDP's consistent domination of the Diet is the campaigning that the New Komeito does on behalf of the government, especially in the House of Representatives’ single-member districts. In this article, we first review the dominant theories on coalition formation and then bring them to bear on the circumstances facing the LDP and the New Komeito parties in an effort to explain why they cooperate. Lastly, we use the Diet's electoral data to trace the effect of the New Komeito’s partnership in the government.