REFILE-UPDATE 5-Japan calls snap election as new party roils outlook

(Fixes headline to make clear poll date has been set)

* Lower house dissolved, election on Oct. 22

* Tokyo Gov Koike's party gains momentum

* Main opposition Democratic Party to back Party of Hope

* Voters may be willing to take a risk - analyst

TOKYO, Sept 28 (Reuters) - Japanese Prime Minister Shinzo Abe on Thursday called a snap election as the main opposition Democratic Party threw its support behind a fledgling party led by Tokyo's popular governor, Yuriko Koike, in the Oct. 22 vote.

Abe, a conservative who returned to power in 2012, hopes a boost in voter support in recent months will help his Liberal Democratic Party-led (LDP) coalition maintain a simple majority. It now holds a two-thirds "super" majority.

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 throw their "full support" behind Koike's group, not run any candidates of their own and allow their members to 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.

A number of opposition lawmakers boycotted the session to dissolve the lower house, 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 defence 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.

A survey by the Mainichi newspaper showed 18 percent of voters plan to vote for Koike's Party of Hope, compared to 29 percent for Abe's ruling Liberal Democratic Party (LDP).

An Asahi newspaper poll showed 13 percent planned to vote for Koike's 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 Koike has staked out different stances on two issues likely to appeal to voters.

She 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 revenue 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 and Hyun Oh; Editing by Michael Perry)