Mori, his popularity in tatters after controversial remarks reviving wartime memories, earlier dissolved the lower chamber in which his three-way ruling bloc holds a massive majority. The lawmakers' terms would have been up in October.
The June 25 date, which is the birthday of the late prime minister Keizo Obuchi, who died last month after a massive stroke, appeared to have been chosen to try to capitalise on possible sympathy votes.
But that advantage may have evaporated amid the political storm sparked by Mori's emotive remarks calling Japan a "divine nation with the emperor at its core." The remarks were an uneasy echo of a state religious ideology that once saw the emperor as a living god in whose name the Imperial Army conquered much of Asia in World War Two.
Many analysts expect Mori's three-way ruling coalition to hang onto a majority -- albeit a smaller one -- but some add that a defeat cannot be completely ruled out.
The ruling coalition linking Mori's conservative Liberal Democratic Party (LDP), the Buddhist-backed New Komeito Party and the tiny New Conservative Party currently holds 336 seats in the 500-member Lower House. The number of seats will be reduced to 480 as a result of legislative changes.
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