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When the Japanese public voted out the Liberal Democratic party last August, its main objective was to end its 50-year grip on power and to give the Democratic Party of Japan a chance to get the country out of a rut. But political analysts, both foreign and domestic, thought they detected another motive. That was the creation of a two-party system that would enfranchise a hitherto passive electorate by giving it a genuine opportunity to choose between competing ideologies and – at intervals rather shorter than half a century – to get rid of underperforming governments.

If that really was the aim, then it does not seem to be panning out. The proximate cause is that the LDP, deprived of the unifying force born of power, is in danger of disintegrating. Far...