Four years of conservative-Socialist cooperation as a ruling bloc came to an end Monday after the Social Democratic Party formally broke from the tripartite alliance with the Liberal Democratic Party and New Party Sakigake.

SDP head Takako Doi met in the morning with her alliance counterparts, Prime Minister Ryutaro Hashimoto of the LDP and Masayoshi Takemura of Sakigake, to inform them of the decision. "As of today, the tripartite alliance is over," Takemura said at the meeting.

The breakup is unlikely to have a significant impact on the national political landscape. The LDP has already regained a simple majority in the Lower House. Although it still lacks a majority in the Upper House, the SDP has pledged to cooperate on pending legislative matters submitted to the current Diet session.