Hi! On the technicality since it was worded with “out of power”, it might actually be only four years.
In 1993, a snap election was called after several LDP members left the party due to derailed electoral reform. The background was a series of corruption scandals and one of the major reforms against corruption was the electoral system reform. See the Sagawa Express and Recruit Scandals.
The electoral multi-member districting system was just to easy to get elected and remain an incumbent leaving policy competition useless. Without the several members, it put the LDP coalition into the minority. The Socialists tabled a motion of no confidence in the government. The motion passed with the LDP rebels. Then the Diet dissolved and elections were set.
The LDP basically lost the snap election in 1993 because they had fewer members due to the rebel ex-members, but no further losses overall. This ended the so-called ‘1955 system’ where political plurality pact of the LDP essentially fell apart. So the rebels composed of Hosokawa’s Japan New Party and the Japan Renewal Party. Even though LDP was still the largest party like 40 percent, they couldn’t form a government without courting several other parties.
The opposition formed a coalition for about one year, but it fell apart. This included Hosokawa’s New Party, the Japan Renewal Party, the Japan Socialists, Social Democrats, and the New Komeito. The Communists were left out. Hosokawa was elected PM even his party was clearly only a small fraction of the coalition.
The coalition fell apart due to infighting on policy. Their biggest priority was electoral reform, but there wasn’t a unifying vision on how. Yet other policies needed attention like economics and trade, which created more riffs and applied strain on Hosokawa’s administration to govern. So they submitted a reform proposal to the Lower House, but they were not able to muster it through during their reign.
Ironically, Hosokawa was accused of his own political scandal and was forced to resign. In addition, he didn’t really want to be PM, but wanted to clean up politics. The PM was taken over by the Japan Renewal Party’s Hata, another LDP rebel. So Hosokawa and Hata were in the PM position for a combined 327 days.
So the Socialists broke away from the coalition and negotiated a coalition with their long-time rivals, the LDP.
So technically, the Japan Socialist Prime Minister of Tomi’ichi Murayama from 1994 to 1996 was rather controlled by the LDP and bureaucrat structures. So the LDP was actually in power, but not in the prime minister position. The LDP could have elected their own prime minister, but compromised on Murayama to push a few reforms that some LDP members could not agree to. The electoral reform was a bit watered-down when it eventually passed. It was a rather fair reform, but it did not really change the distorted incentives.
Murayama stepped down due to failure to win the Upper House. He gave the reigns to the LDP with Hashimoto becoming PM. Murayama was only PM for one year and 196 days
Then in the 2009 election, the Democratic Party of Japan (formerly composed of the Socialists, Social Democrats, LDP rebels, and other parties) was able to limit the LDP to under about 40 percent of the electorate. The DPJ governed for a thorny three years and a few days.
So this might be in total at just under four years where the LDP was out of power. But this is still minuscule compared to the 63 years of LDP reign.
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Anyway, that was longer explaining a technicality. So I wrote about this topic in another post below.
Summary: The LDP was formed at the right time with supposed the secret backing of the US. The electoral map and institutions were advantageous for the LDP. The governance structures created rural prefectural fiscal dependence on the central government. The countryside rural areas being left behind by development had a lot of electoral power. Fiscal supports and other vote buying policies like agricultural subsidies and constructive projects were used to garner support. This might be called ‘clientelism’. In addition, the bureaucratic structures reinforced the dependence by issuing carrots and sticks. Then this weakened the opposition groups that only find support in urban areas, and thus why they are not competitive in the countryside. As time passed, the lack of governance experience has made the opposition parties eternally suspect and the historical shortcomings of opposition governance bolsters the argument as a pervasive sentiment. In addition, there is a lack of charisma and recruitment disadvantage.
So the overall main point is that electoral system and political structures have created fragmented incentives. So despite the LDP governing most of the post-war, it is not a very unified party with several factions that can seem like heavy partisanship. It just so happens their big voter base remains beholden to the individuals in the party. It has made Japan very hard to govern, create policy, and react decisively. The prime minister and politicians are not all that powerful. The bureaucracy and big business are like shadow factions of the LDP that have dark consensus processes. The bureaucracy is extremely different and they might actually run Japanese ‘democracy.’
For sources, please see the link in the middle of post.