He vowed to restore voters' faith in his ruling party.
Polls show his party, the Liberal Democratic Party, is in danger of a historic defeat.
A victory by the opposition Democratic Party of Japan would end more than 50 years of near-unbroken rule by the conservative LDP.
'This is a major, revolutionary election to allow politicians to take the lead in Japanese government,' Democratic Party leader Yukio Hatoyama told party members.
'We should face it with a sense of historic mission.'
Opinion polls show the Democrats well ahead of the LDP among voters, however close to 30% are still undecided.
Mr Aso's announcement of his election plan last week, one day after his party was beaten in a closely watched Tokyo assembly poll, sparked chaos in the LDP, with critics trying to oust him from the top party post.
Party heavyweights blocked the move, but agreed Mr Aso should appear at a meeting of the LDP.
In remarks carried live on nationwide TV, Mr Aso apologised for his failings and admitted that the party's internal chaos had contributed to recent local election losses.
Mr Aso, the 68-year-old grandson of a prime minister, took office last September and has seen voter support slide.
The Democrats have had their own troubles, with one leader forced to resign in May.
