Komeito, with its 19 upper house seats, is the only other than the LDP that can ensure an upper house majority by itself in a coalition with the DPJ (not to mention hat would then become a superfluous lower house majority). The policy inclinations of Komeito’s urban, increasingly middle-class Soka Gakkai constituency is relatively easy to accommodate within the DPJ’s policy agenda. So expect a lot of flirting and dancing to go on between the two; otherwise, government will grind to a stop*. But this does not mean that the two parties will be in a position to enter into a formal relationship in the near future.
* Note that the DPJ can pass the budget by itself, but it won’t be able to account for half of it unless it can muster enough votes in the upper house (or a supermajority in the lower house to override its weaker counterpart) so that it can pass the authorization legislation for the multitrillion JGBs needed to cover the revenue shortfall.Komeito went into the 2009 lower house election with 31 seats and came out with only 21. The loss of all eight incumbents in single-member districts to DPJ or DPJ-supported candidates was particularly keenly felt, since it wiped out the Komeito lower house leadership; not one of the incumbents had been placed on the corresponding regional district list as insurance against a loss in the single-member election. Getting those seats back will the most important part of the asking price—there’s also the upper house and local elections and, yes, policy issues—for a formal coalition.
To accommodate (my hypothetical) Komeito demands, the DPJ would have to force the incumbents (seven DPJ members and Yasuo Tanaka, the maverick New Party Nippon chief) to step down. It might be able to placate its own by putting them at the top of their respective regional tickets, but that would put its own regional district incumbents in peril. These are people who put successful careers on hold, or abandoned them altogether, to cast their lot with the DPJ; they will not go gently into that good night. My guess is that it will take at least another lower house election for the Komeito-LDP relationship to fully unwind—and the DPJ to take Komeito home—if the DPJ and LDP remain in their current configurations if not exact dimensions.