Abe is seeking to establish support to use strong diplomacy on North Korea. But Tokyo's popular governor has a new party contesting in the October 22 election.
Japanese Prime Minister Shinzo Abe officially dissolved parliament on Thursday, effectively kicking off a national election campaign where he faces an unexpected and formidable challenge from the popular governor of Tokyo.
Voters in the world's third-biggest economy will go to the polls on October 22, as Abe seeks a fresh popular mandate for his hard line stance on North Korea and a new tax plan.
"A difficult battle starts today," Abe told reporters, shaking his fist.
"This is an election about how to protect the lives of people," said the premier. "We have to cooperate with the international community as we face the threat from North Korea."
Abe asked for public support for his "strong diplomacy" on North Korea, which has threatened to "sink" Japan into the sea and fired missiles over its northern Hokkaido island twice in the space of a month.
A fledgling party led by popular Tokyo Governor Yuriko Koike gained momentum on Thursday ahead of an October 22 election as the biggest opposition Democratic Party said it would step aside to let its candidates run under her reformist banner.
Abe, a conservative who returned to power in 2012, hopes a recent boost in voter support will help his Liberal Democratic Party-led (LDP) coalition maintain a simple majority. It now holds a two-thirds "super" majority.
But Koike's new Party of Hope, formally launched on Wednesday, has upended the outlook for the election after the former LDP member announced she would lead it herself.
Democratic Party executives said they would not run candidates of their own and let members run under the Party of Hope banner.
The party has struggled to overcome rock-bottom ratings, defections and an image tainted by its rocky stint in power from 2009 to 2012.
According to a poll in business daily Nikkei less than a week ago, 44 percent of Japanese planed to vote for the LDP, while only eight percent favoured the main opposition.
But the emergence of Party of Hope has transformed the political landscape and new polls show Abe's lead has been cut dramatically.
Koike, 65, who became Tokyo governor in a landslide last year, has said her party aspires to offer an alternative to the long-governing LDP and a break from old-school politics and links to vested interests.