By Linda Sieg
TOKYO (Reuters) - A stunning defeat for Japanese Prime Minister Shinzo Abe's ruling party at the hands of a novice political group in the capital has revealed the fragility of his support and shown that voters can desert him - if there is a credible alternative.
Abe surged back to power in 2012, pledging to revive the stale economy and bolster defense. He has led his ruling bloc to three more landslide victories in national polls since then.
But those victories were less robust than met the eye, since record or near-record low voter turnout allowed Abe's Liberal Democratic Party (LDP) to rack up seats with support from a quarter or less of eligible voters.
On Sunday, the party got a chilling glimpse of just how vulnerable it could be if a viable opposition force emerges to attract unhappy voters. That reminder could encourage rivals in Abe's own party to challenge him in a leadership race next year if his slumping poll ratings fail to recover.
Popular Governor Yuriko Koike's novice Tokyo Citizens First party and its allies - including the LDP's national-level coalition partner - won a landslide victory, taking 79 seats in Sunday's election for the 127-member Tokyo Metropolitan assembly. The LDP got 23 seats, its worst ever result in the capital and less than half its pre-vote presence.
The vote was a referendum on the first year in office of Koike, a media-savvy former LDP lawmaker and defense minister who defied the party's old boy network to run on a reformist platform and become the first female governor of the metropolis.
But it was also a stinging rebuke to Abe's administration, battered by suspicions of scandal over favoritism for a friend's business and verbal gaffes by his cabinet ministers.
Worse than the scandals and missteps, however, was the perception among many voters that Abe and his powerful Chief Cabinet Secretary Yoshihide Suga have grown haughty and dismissive of criticism given the ruling bloc's super-majorities in both houses of parliament, a weak and fragmented opposition and no serious challengers in his party.
"Arrogance, complacency, fragility" summed up a headline in the Nikkei business daily on Tuesday, ingredients politicians and pundits agreed were key causes of the LDP defeat.
"It doesn't have anything to do with policy," said Gerry Curtis, professor emeritus at New York's Columbia University.
"It's all about the arrogance of Abe and Suga and the sense that they are riding roughshod ... to break unstated rules of the game."
Abe heads to Hamburg on Wednesday for a Group of 20 summit where leaders are likely to discuss North Korea's nuclear and missile programs - a topic that could play to Abe's strengths at home, where he is known for his tough line towards Pyongyang.
North Korea said on Tuesday it had successfully test-launched an intercontinental ballistic missile that could strike anywhere in the world.
In an interview with the Mainichi newspaper published on Tuesday, Abe said he would "reflect deeply" on the Tokyo poll results.
But he also said there would be no change to his plan to have the LDP present a proposal to revise the post-war, pacifist constitution to an extra session of parliament in the autumn to meet his target of amending the U.S.-drafted charter by 2020.
Trying to stick to that timetable for what would be a deeply divisive amendment, however, could be risky, since even many who support the change don't see it as an urgent priority.
Abe's conservative agenda of restoring traditional values and loosening constraints on the military centers on revising the constitution. But the public are deeply split on the proposal, which would antagonize China and South Korea.
The huge win for Koike's novice party has fanned speculation that her group will go national in a general election that must be held by late 2018, perhaps joining with disaffected lawmakers from the struggling opposition Democratic Party.
The Democrats, who have had little success in repairing their image after a rocky three-year reign ended in 2012, also fared badly in the Tokyo poll, taking only five seats.
A new party could set up Koike for a run at the nation's top job, but her allies have said she's unlikely to quit as governor to return to parliament before the 2020 Tokyo Olympics.
In the meantime, with much of Abe's previous clout over his party based on his record of leading it to election wins, the dismal Tokyo showing could well encourage rivals to challenge his bid for a third term as LDP leader from September 2018.
Victory in that party poll would put Abe on track to become Japan's longest-serving leader, eclipsing early 20th Century premier Taro Katsura who logged nearly eight years in power.
Among those floated as possible challengers are former Defense Minister Shigeru Ishida, who has been focusing on ways to revitalize Japan's depopulated rural regions, and Foreign Minister Fumio Kishida, considered more dovish than Abe.
"As politicians realize that Abe is vulnerable, that will inspire them to do something," Curtis said. "He'll probably survive, but even so, I don't think he will get much done in terms of policy."
(Reporting by Linda Sieg; Editing by Alex Richardson)