Thanks to Aurelia and others for contributing to an interesting discussion about why the DPJ is not able to maintain cabinet (or party) discipline. I agree that it is more than a coordination or PR problem at the top, although I am sure that better efforts there would help.

Since Japan's electoral and parliamentary structure now resemble Britain's so closely (Margarita Estevez-Abe describes Japan as having moved to a "Westminster System"), I do not think we can easily blame the difficulties on institutions. It has now been 17 years since Japan moved away from the old electoral system that fostered factionalism.

I think an under-appreciated factor is the centrism and lack of clear policy cleavages among the voters and between the parties. An important reason why the Labour and Tory Parties maintain discipline is because there are big policy differences between the parties and between voters over economic policy. If the Tories split or splinter, they leave the door open to a Labour Party that will implement quite different policies.

Under the 1955 System and during the Cold War, a similar policy cleavage over security policy held the LDP together, despite factionalism, and even held a never-winning JSP together. Since then, the two main parties have alternated between competing to be seen as reformist (Koizumi LDP v shrink-the-govt DPJ) and later competing to be seen as bolstering the welfare state.
We have yet to see an election, much less a stable and enduring debate, emerge between one party advocating neoliberal reforms and the other advocating a bigger welfare state.
Today, I would have a hard time specifying where the policy debate is between Noda and Tanigaki, other than the artificial short-term differences that arise from political-posturing. We have tended to see the muddle as driven by the broad-range-of-opinion within each party, but I think it is at least in part a product of the absence of clear divisions or debates within the electorate.

Is it possible to get disciplined parties when the policy differences are this low?

Len Schoppa