* Lower house dissolved, election on Oct. 22
* Tokyo Governor Koike's party gains momentum
* Main opposition Democratic Party to back Party of Hope
* Voters may be willing to take a risk - analyst
* Koike to weigh steps to exit nuclear power by 2030 (Adds Koike comment on nuclear power, PM Abe comment)
TOKYO, Sept 28 (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 conservative, 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, only formally launched on Wednesday, has upended the outlook for the election after the former LDP member announced she would lead it herself.
"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 media-savvy former defence minister whose name has often been floated to be Japan's first female prime minister, Koike said she would not run for a seat herself, although speculation persists that she will.
Democratic Party executives said they would not run candidates of their own and would 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.
After the cabinet formally set the date of the election, Abe told reporters, "I decided to call this election because we must overcome the national crisis of the threat from North Korea and an ageing population by obtaining a mandate from the people."
Some opposition lawmakers boycotted the dissolution session, in protest against Abe's election decision, which could bring about a political vacuum at a time of high tension with North Korea over its missile and nuclear arms programmes.
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.
TAKING A RISK
"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, who wants Japan to abandon nuclear power, said she would study ways to eliminate dependence on it by 2030. Abe's government aims to retain its role in the energy mix, despite worries over safety after the 2011 Fukushima nuclear crisis.
Koike also 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.
Recent reforms will cut to 465 from 475 the number of lower house seats in the coming election. (Additional reporting by Elaine Lies, Noriyuki Hirata, Hyun Oh and Tim Kelly; Editing by Michael Perry and Clarence Fernandez)