When Yuriko Koike split from the Liberal Democratic Party in May 2017, 10 months after she had been elected governor of Tokyo, Prime Minister Shinzo Abe could have been forgiven for assuming that she could be relied upon to support the national government in at least some policy areas.

After all, she had served with the LDP for 15 years, held three cabinet portfolios – including that of minister of defence under Abe – and is widely thought to hold conservative views similar to the prime minister’s on many issues. Koike’s crushing victory in elections for the Tokyo municipal government in July, however, would have finally disabused the prime minister of any notion that cooperation might be possible.

As the head of the Tomin First no Kai movement, which translates as The Tokyo Residents First Association, Koike took 79 of the 127 seats in the municipal assembly, with the LDP winning a mere 23 seats – its worst ever showing in the capital and down from 57 seats before the vote.

And throughout the campaign, there was never any hint of Koike doing anything other than going for the LDP’s jugular.

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Since that win, Koike has momentum behind her and is playing on the voting public’s disillusionment with the present state of Japanese politics. The opposition is disorganised, fragmented and lacks coherent proposals that offer alternatives to the LDP’s policies.

Analysts say it is very possible that the Democratic Party, at present the largest single opposition party, will fracture along factional lines if it fares badly again in the upcoming election. As a consequence, a lot of voters are putting a cross in the LDP candidate’s box when it comes to election day simply because there is no viable alternative.

They may disagree on Abe’s plans to rewrite the constitution, oppose anti-conspiracy legislation – which critics have charged could be used to violate freedom of expression and the right to privacy – and be singularly unimpressed with efforts to invigorate the economy, but many reluctantly go along with the adage “better the devil you know”.

Koike now offers those wavering voters a viable alternative, and she has come out swinging in the early days of the election campaign.

On Monday, announcing the founding of a new political movement that she will head, Kibo no To, or Party of Hope, she took aim squarely at the prime minister and his track record.

“Japan is talking about reform while its presence is declining,” she told reporters at the launch of the new party. “Can we really leave things up to them?”, she said in a dismissive reference to the LDP.

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She took issue with sluggish economic growth, Abe’s plans to rewrite the constitution and how the government intends to spend the windfall when the consumption tax is raised to 10 per cent in 2019 – as well as vowing to do away with Japan’s nuclear power plants.

Koike’s grip on the opposition was further underlined on Thursday, when Seiji Maehara, the head of the Democratic Party, announced it would join forces with Koike’s fledgling movement – effectively disbanding the largest opposition party and handing Kibo no To a major boost, including the 80 seats that it holds in the chamber.

Koike is effectively depicting the LDP as a faceless, monolithic presence that achieves nothing, while her party is the everyman that can be relied upon to bring change. She has weeks to communicate that to a public that is listening and, broadly, is desperate for an alternative.