So how does Mr. Abe stay in power when his policies are so unpopular?

The secret of his grip has been, largely, the lack of an alternative. The centrist Democratic Party, the country’s main opposition party, was discredited after displaying inexperience and incompetence during its brief, and only, stint in government, under a former name, in 2009-12.

After the nuclear accident at Fukushima in 2011 spurred renewed activism at the grassroots level, the Democratic Party formed an alliance with civil society groups and smaller leftist parties. The strategy paid off, and led to substantial gains in the 2016 upper house election. But it also came at a price, namely growing resentment among Democratic Party conservatives. And even the party’s inroads couldn’t compensate for one of the opposition’s most enduring weaknesses: its lack of a convincing leader. The conservative Seiji Maehara was elected as the Democratic Party’s new head on Sept. 1, but, perceived as a has-been, he hardly has enthused the public.

Enter Yuriko Koike, the populist and media-savvy governor of Tokyo, whose party handsomely defeated Mr. Abe’s in the Tokyo metropolitan assembly election in July. Last month, on the very day that Mr. Abe announced the dissolution of the Diet’s lower house, Ms. Koike inaugurated a new party, the Party of Hope, with great fanfare. She then promptly struck a surprise deal with Mr. Maehara under which, in essence, the Party of Hope would take over the Democratic Party. For a moment, Ms. Koike appeared to be revving up to become a direct challenger to Mr. Abe.

But as the campaign period was about to begin, Ms. Koike announced, without much explanation, that she would not be running. That decision was another victory for Mr. Abe, and another one handed to him by his own rivals: By then, Ms. Koike had, in effect, already killed the liberal-left alliance.

Ms. Koike claims to have created the Party of Hope in order to “reset Japan.” It’s a conveniently vague slogan — and reminiscent of the Liberal Democratic Party’s own, “Take Japan Back.” The Party of Hope rests on a policy platform of catchy but vague sound bites — it is against nuclear energy, overhead power cables and hay fever, among other things — some fundamentally at odds with Ms. Koike’s avowedly conservative positions.

She hardly is an ideological foe of Mr. Abe’s: She served in his first government, including as defense minister in 2007. Which is one reason that when Ms. Koike decided not to run in the upcoming Diet race, it suddenly seemed as though she had never intended to take on Mr. Abe but rather had been positioning herself to strike a deal with him after the election.