On September 9, the DPJ agreed to form a coalition government with the Social Democrat Party (SDP) and Kokumin Shinto. The DPJ needed to form a coalition government because, despite winning an overwhelming majority in the Lower House, they still did not have a majority of the seats in the Upper House.
In reality, for Japan, this coalition government scheme was a form of the good cop / bad cop negotiating scheme. The DPJ would be the good cop, the party who wanted to be cooperative with America. The other two parties, the SDP and Kokumin Shinto, would be the bad cop. The SDP would be the bad cop in regards to foreign policy and security issues while Kokumin Shinto would be the bad cop in regards to economic issues. The SDP was the successor party to the Japan Socialist Party (JSP) and was the party who wanted to relocate Marine Corps Air Station Futenma outside of Okinawa. Kokumin Shinto basically had an anti-neoliberal approach to economics, meaning they had an anti-Koizumi approach and an anti-Washington Consensus approach to the Japanese economy. They opposed the privatization of Japan Post and, whenever the opportunity presented itself, they demanded that Japan enact an impossibly large fiscal stimulus plan (which would mean that the American government would have less money to spend as Japan funds a significant portion of the U.S. budget deficit).