Over the long-run, the Japanese legislature mainstream will only accommodate one or two large political parties plus Komeito over the long run. Smaller mainstream parties must go big quickly, by merger or endogenous growth; otherwise, they will wither and go away. Let me explain:
In an industrialized economy, it is difficult to carve out a set of distinct policies that appeal to the political mainstream across the wide range of issues that command public attention. Mainstream parties tend to look in substance like modest variations of each other. Moreover, the Japanese electorate is relatively free of religious, ethnic, tribal, historical and other enduring, visceral cleavages that allow small political parties to operate and thrive in the policy mainstream. This means that mainstream voters and prime candidate material will gravitate over the course of multiple elections towards parties that have a reasonable chance of capturing, then dominating the policymaking and implementation process. That means the ruling party/coalition, plus at most one dominant opposition party. The combination of one or more charismatic political figures and destabilizing circumstances may generate new parties and breakouts, but those creations will dissipate over the long run if they cannot outgrow their initial respective raisons d’ être and become the next big thing.
The LDP will maintain its position as a dominant mainstream party by virtue of its 1/4 bedrock share of the voting electorate plus an enduring coalition with Komeito (and its 1/10 bedrock support base) that includes intimate coordination at the SMD level (making Komeito the virtual pacifist-wing of the LDP). Moreover, the coalition has a House of Representatives supermajority that must be used sparingly from a media-management perspective but will enable it to pass annual tax legislation, which, coupled with the ~FY2012, blanket deficit-bond authorization, will enable it to keep the government running without regard to the configuration of the House of Councilors. This means that if the LDP does badly in the 2013 HoC regular election, it can jettison Abe in favor of a baby face and continue in power without calling a HoR election until December 2016, when its current term ends. The outcome of the 2013 HoC election is crucial for Abe’s long-term survival as prime minister, but is only a speed bump for the LDP.
The DPJ, which was outvoted by the JRP in the proportional reginal districts in the December 17 House of Representatives general election, is in worse shape. A solid, if not bedrock, 3/20 support base still appears to be in place, but it is significantly smaller than the LDP support base and has a much shorter history. Then there’s the Komeito/Sokagakkai vote, which is like spotting the LDP a 10-meter handicap in a 50-meter race. In other words, the DPJ is highly reliant on the volatile mainstream floater vote, which currently has two attractive alternatives on the Third Force movement front: the Your Party—and the Hashimoto-Ishihara JRP. If these two can reconcile their mostly non-policy differences and coordinate their political efforts in the 2013 HoC election, they could overtake the DPJ as the dominant mainstream alternative. If the DPJ’s performance under these circumstances is anything near the most recent HoR results, it will wind up a distant third, and likely pulled apart in the not-too-distant future by the gravitational pull of the two dominant mainstream parties. Thus, in the case of the DPJ, the 2013 HoC election will be an existential battle for its survival.