Koizumi has long insisted that he will step down when his term as head of the ruling party expires in September, and the conservative Aso had been expected to announce a bid, the AP reports.
Aso aide Takashi Sasaki said a formal announcement of Aso's candidacy would be made at a yet-to-be-decided date, after getting formal backing from the 20 lawmakers needed to put his name on the ballot.
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