The Liberal Democratic Party (LDP) held power from 1955 until 1993. How did it manage to do so? In 1994 a political reform resulted in competitive elections but, starting in 2012, the LDP regained its predominant position, winning three consecutive landslide victories. How did it manage to do this even after the reform? In this paper I argue that a system of ‘party-organization patronage’, in which the patron is the LDP and the client is an interest group organization, played a significant role in maintaining LDP predominance in both periods. I further argue that the key to explaining changes in the predominant party system is the LDP’s monopoly on access to the public policy making process. When this monopoly faltered, interest group organizations began to put a foot in both camps or even defected to an opposition party but once the LDP’s monopoly was re-established interest groups returned to the LDP fold. In both cases, predominance was established in two steps. First, fortuitous events gave the LDP an overwhelming parliamentary majority and a monopoly on access to policy making. Second, the LDP granted organizations access to policy making in exchange for their votes which helped it maintain its monopoly.