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He was elected to the House of Representatives for the first time in 1990. After losing his seat in 1996, he ran unsuccessfully for the House of Representatives in 2000. In 2001 he was elected to the House of Councillors for the first time. He became the party's leader on 8 September 2009 after the party suffered a major defeat In the 2009 Japanese general election. New Komeito lost ten seats, including that of party leader Akihiro Ota and general secretary Kazuo Kitagawa. On 8 September 2009, Yamaguchi replaced Ota as president of New Komeito.