Correction: An earlier version of this article misspelled the name of a nuclear expert who quit as an adviser to Prime Minister Naoto Kan in April. His name is Toshiso Kosako. This version has been corrected.

About 10 weeks ago, Japanese Prime Minister Naoto Kan started acting differently — less as a part of the system than as somebody intent on battling it.

He first issued a vague promise to resign. He then undertook what amounted to a full character change, awakening as a reformer just as the country signaled its dissatisfaction with the job he was doing. As part of this change, he announced policies without consultation. He torched links with former supporters. He withdrew from the press, spending more time posting to his personal blog. He crusaded against nuclear power and ­blasted those who sought to protect it.

The push for a nuclear phaseout has represented Kan’s last chance to influence Japan’s future energy policy and upgrade his own legacy. Supporters and critics agree he has accomplished the former — advancing the popular anti-nuclear cause despite his own deep unpopularity. The latter goal is more in doubt. Elsewhere, perhaps, Kan’s fist-shaking departure would have galvanized personal support; in Japan, it has drawn mostly scorn. Kan, critics say, has taken on a task normally reserved for a group.

“We have decision-making protocols in this country,” said Toshiso Kosako, a nuclear expert who quit as an adviser to Kan in April. “But Kan’s position is to directly ignore those procedures.”

Last week, Kan gave the clearest indication yet that he will resign, pending the expected passage in parliament this month of two bills — a development he had set as a condition. Once he leaves, the ruling Democratic Party of Japan will elect its new president. That president becomes the next prime minister — Japan’s seventh in five years. No candidate so far has taken the same hard anti-nuclear stance as Kan, but widespread public opposition to nuclear power ensures major obstacles if the government tries to restart its idled reactors.

When a 9.0-magnitude earthquake struck March 11, triggering a tsunami that devastated much of the northeastern coast, Kan was on the verge of being forced from office. The disaster gave him a reprieve. But the ensuing nuclear emergency at Fukushima Daiichi guaranteed that the reprieve would not be clear-cut. As critics railed at the government for obfuscating data, Kan became a critic himself. He saw a government packed with nuclear advocates, too cozy with the electricity industry.

On April 20, Kan invited four or five lifelong friends to his residence. His wife, Nobuko, served sashimi. Kan had one or two glasses of wine and told his guests about his trip to the crippled Fukushima plant, commenting, according to one guest, that “the nuclear power policy in this country is messed up.”

Naoki Tanaka, a civic activist who attended the dinner, offered a personal interpretation of his host’s thinking. “When Kan later announced his intent to resign, he must have thought about his legacy,” Tanaka said. “At that time he had none. But he decided that steering Japan to rely less on nuclear power was his historical mission.”

In early July, a month after saying he would resign on an undisclosed date, Kan announced that Japan’s idled reactors would be subject to stress tests before they could be re­started. The decision, reportedly made without consultation, divided his cabinet and bewildered communities that had received official assurances of the reactors’ safety.

On July 13, Kan said at a news conference that Japan should eventually eliminate its reliance on nuclear power. The statement was his boldest yet, but with one problem: It represented his opinion, not official government policy. Opposition lawmakers and other critics denounced it as irresponsible, forcing Kan to explain on his blog that “full-fledged discussions” on Japan’s energy policy could come after he had shown a clear path forward.

“Even if Kan did spell out the proper nuclear power policies,” Hiroshi Tasaka, a special adviser to the prime minister’s office, said in an e-mail, “the media waged a negative campaign on him and portrayed his policies as something he came up with off the top of his head.”

The latest polls conducted by major Japanese newspapers show that Kan’s approval rating has dropped below 20 percent. The same polls show that more than 70 percent of Japanese support a nuclear phaseout. Some politicians, including members of his own party, have interpreted the prime minister’s anti-nuclear stance as a bid to hang onto power for as long as possible.

But for his remaining supporters, the past few weeks have shown Kan at his best — liberated from daily political concerns and acting with conviction, the way many felt he did during a stint as health minister in the 1990s, when he investigated a scandal in which hemophiliacs received HIV-tainted blood.

“People often criticized the prime minister for wanting to stay in power or for trying to stay in power as long as possible,” Tasaka said. “I felt the opposite way. Because he had no attachment to his power, he could articulate whatever he wanted to say and boldly act on his words.”

Special correspondent Ayako Mie contributed to this report.