The purpose of this study is to trace the transformation of electoral alliance between Japan’s Liberal Democratic Party (LDP) and Kōmeitō over the past two decades. LDP’s cooperation with the Kōmeitō has allowed the party to sustain its dominance amid disintegration of political foundations in the 1990s, and it was particularly the electoral vulnerability in urban regions which LDP sought to compensate through the integration of Kōmeitō votes. While conventional assessments tend to define the LDP-Kōmeitō electoral cooperation as preprogrammed, commensurate exchanges of votes during general elections, however, this study highlights the unequal distribution of electoral resources as the essential component of ‘cooperation.’ The LDP and Kōmeitō alike developed such a system that allows individuated incorporation of Kōmeitō votes on the one hand, and the Kōmeitō devised an internal mechanism to avoid over-supporting the LDP counterpart, on the other. Such electoral alliance characterized by flexible ‘adaptation mechanism’ not only provides resilience against LDP’s electoral vulnerability in the urban regions but also has allowed the two alliance partners to overcome policy and ideological incompatibilities. To illustrate how such ‘adaptation mechanism’ functions during the executions of electoral cooperation, this study analyzes the past six general elections held between 2000 and 2014. The transformation of the LDP-Kōmeitō alliance from discord to integration, then to distraction, reveals that the sustainability of the LDP-Kōmeitō coalition alliance was engineered not only by the high level of coherence among Kōmeitō supporters, but also through the alliance’s ability to flexibly adapt to changing internal and external environments.