民主黨於2009年的眾議院選舉中大勝，取代自民黨執政，然而也在3年後的選舉中大敗而結束政權。「民主黨政權為何失敗？」成為研究現代日本政治變動的新題材。既有的研究主要從政治主導的失敗、首相領導權的不足，以及政權公約的挫折來分析，本文則從「分立國會」的新觀點來分析，論述在「分立國會」架構下，參議院如何透過「特例公債法案」的審查，來影響民主黨政權的重要政策與政權運作。本文研究結果發現，以往被視為眾議院輔助角色的參議院，其影響力已經不遜於眾議院。在「分立國會」的架構下，朝小野大的參議院透過「特例公債法案」的審查，對執政黨的預算案、重要法案、內閣的更替均具有關鍵的影響力。事實上，從野田首相宣布解散眾議院來看，參議院更有影響眾議院任期以及政權輪替的影響力，參議院已然成為研究日本政黨政治不可忽略的題材。

The Democratic Party of Japan (DPJ) won a landslide victory in the election of House of Representatives in 2009, replacing the Liberal Democratic Party (LDP) as the ruling party, but abruptly terminated due to an overwhelming defeat in another election three years later. Why did the DPJ fail? This question has become a striking issue of modern political science in Japan. Existing studies mainly focus on perspectives such as defeat of political dominance, inadequacy of prime minister's leadership, or failure of Manifesto. This article aims to analyze the role of the House of Councilors in policy-making and manipulation of DPJ in a divided Diet. The House of Councilors used to be regarded as the supporting role; however, this study finds that its influence was not inferior to that of the House of Representatives. In a divided Diet, the House of Councilors exerted more influence on budget bills, important policies, and even cabinet turnover during the review of Bills Funding Special Deficit-financing Bonds. In fact, the dissolution of the House of Representatives declared by Prime Minister Noda gave even more influence to the House of Councilors, especially on term limit of House of Representatives members and alternation of political power. The role of the House of Councilors have become an increasingly important topic in Japanese political studies.