The dominance of the LDP is due to many factors. During the ‘55 System (1955-1993), the electoral system in Japan was different than it is now. At that time it was a Single Non-Transferable Vote, multiple-member district system. To understand how the LDP took advantage of the system I’ll break down how it works. In SNTV, the voter gets one vote for their preferred candidate in their district. It’s a plurality vote. The MMD portion means that more than one candidate can win from each district (eg. a particular district will take the top three vote-getters and send them to parliament). Perhaps you can see how this can be manipulated. A party could send multiple candidates to a district and win multiple seats in a district, creating a stranglehold in parliament. This is essentially what the LDP did. How did the LDP avoid its candidates simply beating up on each other? The answer lies mostly in PARC (the policy affairs research council) of the LDP, though other institutions such as factions and koenkai played roles in this. PARC during the ‘55 System was the major policy-making body of the LDP. It also was fairly specialized, being divided into different policy departments. Politicians would be trained in different areas of expertise over time. When sending candidates to the districts, the candidates would focus on different policy areas. The LDP candidates would not really be campaigning against each other in the districts necessarily since they were on different platforms, or at least were not directly attacking each other. Essentially, the LDP was the master of specialized policy-making, employing lots of pork-barrel and just generally catering to specific demographics, especially the countryside. Other factors played into this, but these are the big factors in LDP dominance from 1955-1993.

After various scandals (and other big issues in the late ‘80s and early ‘90s), the LDP finally lost power in 1993 for the first time. After that, an electoral overhaul happened. The SNTV MMD was replaced by a MMM SMD (mixed-member majoritarian single-member district) system. MMM has a much different format. Basically, each voter gets two votes. One for the candidate of their choice in their district, and one for the party of their choice. The party vote is called a PR list vote and is pretty common in democracies. This change in voting system helped to make alternative parties more viable. In fact, after the system was implemented in 1996, there has been an explosion of new parties in Japan. Many die quick deaths, but the main point is that they exist, whereas from 1955-1993 the only opposition was the JSP (Japanese Socialist Party). Elections themselves have changed over time, shifting towards favoring cities rather than rural, and the platforms changing along with that. One would think that the LDP would suffer since it is right-leaning (in Japanese terms of politics) and the cities are generally more left-leaning. However, LDP dominance has persisted.

The big thing to point out here is the fractured state of political parties in Japan since 1994. The LDP has held together for various reasons, but its opposition is extremely diverse. And smaller. The new system is heavily based on coalition governments. LDP has been helped by forming coalitions with different conservative parties in order to maintain a large majority in parliament. Overall, the party has had declining popularity for a very long time. The problem is that no opposition party (with the exception of the DPJ from 2009-2012, when it miraculously won power) has been as desirable even as much as the unpopular LDP. This is a problem for the opposition. At this point it is fairly certain that a big part of why the LDP dominance continues is because the opposition is not coordinated enough in creating coalitions (specifically liberal) in opposition. The LDP can be defeated; their position is extremely precarious compared to the ‘55 System, and under this electoral system they can easily lose a majority. If the opposition can cooperate. Which, generally speaking, they have not done. The LDP’s politics actually have changed over time, but their institutions have kept them afloat, and they have been very good about maintaining important ties with other smaller conservative parties whereas the opposition has seldom united. Another issue is that those opposition parties are often short-lived and do not have name recognition or stability to their name. The DPJ was a disaster and the LDP is smart about playing up the idea that they are the only ones who know how to govern.

Anyways, I am trying to give a bit of a general overview. As for some sources, I think you will find

The Rise and Fall of Japan’s LDP: Political Party Organizations as Historical Institutions by Ellis S. Krauss and Robert J. Pekkanen to be a great read. It is all about the LDP and how it has shifted over time to remain the dominant party. There is much more to this than what I said above, but I wanted to give a general idea.

Also, for current politics, which is where I pulled a lot of the current ideas, check out

Japan Decides 2017: The Japanese General Election edited by Robert J. Pekkanen, Steven R. Reed, Ethan Scheiner, and Daniel M. Smith