A maverick alumnus of the ruling party has started her own reformist group to challenge the boy's club elite. Tokyo's elections are often seen as a bellwether for national polls.
Tokyo residents were voting on Sunday for the city's assembly in an election that could spell trouble for Prime Minister Shinzo Abe.
Opinion polls predicted a sweeping victory for Governor Yuriko Koike's new party, Tomin First no Kai, or "Tokyoites First" party, as Abe's Liberal Democrats Party (LDP) were buffeted by scandals and gaffes.
Abe is suffering from slumping support after years of stability partly because of a scandal over suspected favoritism.
Tokyo's elections often set the tone for Japan's national polls and Koike was rumored to be eyeing a return to parliament to run for prime minister.
The former newscaster became the first female leader of Tokyo last summer and earned a reformist image after repeatedly clashing with the male-dominated city government.
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She portrayed the LDP-dominated assembly as being an anti-reform old boys' club which was interfering with her agenda, including cost-cutting of the 2020 Tokyo Olympics. She enjoys approval ratings of about 60 percent.
Koike used to be an LDP member and held key party and Cabinet posts including defense minister. She angered party seniors when she abruptly ran for Tokyo's governor last year, but had never officially left the party until last month to head her own.
If Koike's party wins wide support it could fuel speculation that she will make a bid for the nation's top job, though probably not until after the 2020 Tokyo Olympics.
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It could also widen cracks between the LDP and coalition partner Komeito while damaging prospects for the opposition Democratic Party.
Abe's rivals in his party could be encouraged by a dismal LDP performance to challenge him in a leadership race in September 2018, victory in which would set Abe on course to become Japan's longest-serving leader and bolster his hopes of revising the post-war, pacifist constitution.
Gerry Curtis, professor emeritus at Columbia University, told news agency Reuters that Japan's political landscape could be set for a shake-up if Koike's party and its allies win big.
"We may discover that Japan is not all that different from Britain, France, and the U.S. in its ability to produce a big political surprise," he said.
aw/rc (Reuters, AP)