Post-war Japan w/ less-dominant LDP

Ever since the 1950's , the Liberal Democratic Party has had a nearly continuous reign as the dominant Japanese political party - only being out of power in 1993-94 and from 2009-2012. This is due to lots of factors - the LDP itself is a big-tent party with distinct factions, also taking advantage of favorable voter districting and weak opposition parties to stay afloat.

What would it take for Japan to have a more diverse range of governments and prime ministers under its post-WWII system?
 
They rose to power with great help from the US. Look up Kishi Nibusuke and the 55 System.
If they are not to rise to power then the following needs to occur pretty early on, i.e. the early 60s:
1. LDP falls from internal crisis or major corruption scandal
2. Viable centre-right or right party rises for US to support; if leftist, needs to be at least pro-US
3. US supports the said party.
 
Wasn't/Isn't the LDP the rubber-stamp party created to give a democratic pretense to the United States meticulously craft technocracy?
 
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