The Liberal Democratic Party's fourth-largest faction, led by ex-Trade Minister Mitsuo Horiuchi, said Wednesday it will back Ryutaro Hashimoto in the four-way race for the party presidency.
The decision was made during an emergency meeting.
The LDP presidential election will be held Tuesday. Whoever triumphs is almost certain to become prime minister, with the LDP-led coalition holding a majority in the more powerful Lower House.
Aside from Hashimoto, who is looking to serve for a second time, the three other candidates are former Health Minister Junichiro Koizumi, LDP policy chief Shizuka Kamei, and Taro Aso, minister in charge of economic and financial affairs.
Hiromu Nonaka, a former LDP secretary general who belongs to the biggest LDP faction, which is chaired by Hashimoto, told the emergency assembly that if Hashimoto wins, Makoto Koga will probably stay on as secretary general of the party.
Koga belongs to the Horiuchi faction.
Horiuchi criticized Koizumi, Hashimoto's strongest rival in the race, for suggesting he would not necessarily stick with the current ruling coalition if elected LDP president.
The coalition consists of the LDP, the New Conservative Party and New Komeito.