Opposition parties are wary of a possible early general election following the ruling Liberal Democratic Party's leadership vote next month.
They are concerned that the LDP may regain public support after choosing a new leader and opt for a snap election for the Lower House.
Among opposition parties, the Democratic Party for the People (DPP) and Sanseito made gains in July's Upper House election while the Constitutional Democratic Party of Japan (CDP) and Nippon Ishin no Kai were sluggish.
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