By Linda Sieg and Elaine Lies
TOKYO (Reuters) - A fledgling party led by popular Tokyo Governor Yuriko Koike gained momentum on Thursday ahead of an Oct. 22 election as the biggest opposition Democratic Party said it would step aside to let its candidates run under her reformist banner.
Prime Minister Shinzo 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.
"This will be a tough battle, but it's all about how we will protect Japan, and the lives and peaceful existence of the Japanese people," Abe told a group of lawmakers before the cabinet formally set the date.
Some opposition lawmakers boycotted the session to dissolve the lower house earlier in the day, in protest against Abe's calling the election and creating a potential political vacuum at a time when tension with North Korea is high.
Koike, a media-savvy former defense minister whose name has often been floated to be Japan's first female prime minister, said on Wednesday she would not run for a seat herself, but speculation that she will persists.
"I'm someone who is always ready to take action," Koike told a news conference where she spoke about her achievements since taking office as governor a year ago.
A survey by the Mainichi newspaper showed 18 percent of voters plan to vote for Koike's party, compared to 29 percent for Abe's ruling Liberal Democratic Party (LDP).
An Asahi newspaper poll showed 13 percent planned to vote for her party, versus 32 percent for the LDP. Both surveys asked voters their preference for proportional representation districts where ballots are cast for parties, rather than candidates.
"Voters in many countries have shown they are willing to take a risk, even a severe risk, in terms of what will actually happen because they are disappointed with the status quo," said Martin Schulz, a researcher at Fujitsu Research Institute.
But Schulz, who drew a comparison to French President Emmanuel Macron's meteoric rise, added that Koike's platform might not be so appealing, given its similarities to LDP policies.
Abe's personal ratings have risen to about 50 percent from about 30 percent in July, partly on the back of his leadership during the current North Korea crisis.
But opposition parties say he called the election to escape questioning in parliament about suspected cronyism scandals that had cut into his support.
Koike, 65, defied the LDP to run successfully for Tokyo governor last year and her novice local party then crushed the LDP in a metropolitan assembly election in July.
Her Party of Hope shares policy space with the business-friendly LDP, but she has staked out different stances on two issues likely to appeal to voters.
Koike wants to freeze a planned rise in the national sales tax to 10 percent from 8 percent in 2019. Abe says he will raise the tax but spend more on child care and education instead of paying down public debt.
Koike also wants Japan to abandon nuclear power, while Abe's government plans to retain it as a key part of the energy mix, despite safety worries after the 2011 Fukushima nuclear crisis.
Recent reforms will cut to 465 from 475 the number of lower house seats in the coming election.
(Additional reporting by Noriyuki Hirata, Hyun Oh and Tim Kelly; Editing by Michael Perry and Clarence Fernandez)