It wasn’t so long ago that Shinzo Abe appeared on course to becoming Japan’s longest-serving prime minister ever.

But after his conservative Liberal Democratic Party’s crushing defeat in Sunday’s Tokyo assembly election, Abe’s iron grip on power has finally shown signs of slipping — a setback critics say is the biggest crisis he has faced since his sweeping return to power in December 2012.

“This is a very stern rebuke from the public that we must do some serious soul-searching about,” NHK showed a grim-faced Abe telling reporters at his Tokyo office Monday morning.

“It’s been five years since this administration kicked off, and I believe the public has sent us a tough message that we have lost our discipline,” he said.

Although Tokyo assembly elections have often served as a harbinger of national trends, there is no immediate danger of the Abe administration disintegrating, as the next Lower House election isn’t scheduled until next year. Experts say the LDP’s shattering defeat hardly leaves Abe in the mood to call a snap election anytime soon, either.

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But the results bode ill for the prime minister nonetheless and point to the “beginning of the end of Abe’s invincibility,” according to Norihiko Narita, a political science professor at Surugadai University.

Sunday’s election saw Abe’s LDP lose big to a fledgling party headed by the capital’s popular governor, Yuriko Koike. Tomin First no Kai (Tokyoites First), as it is called, trounced the LDP by capturing 49 seats in the 127-member assembly. The tally, if combined with seats won by its allies, including Komeito, gives the pro-Koike forces a comfortable majority in the assembly at 79 seats.

The outcome left the LDP tied with Komeito at 23 seats, followed by the Japanese Communist Party with 19, and the Democratic Party with five.

Asked about factors that may have contributed to the LDP’s rout, Chief Cabinet Secretary Yoshihide Suga said the “lost discipline” of the Abe administration and a recent string of “inappropriate remarks” by LDP lawmakers likely fueled voter anger.

In March, the LDP rubber-stamped a historic rule change to extend the maximum tenure of its president to nine years, up from six, in what was seen as a move to enable a third term for Abe that would continue through 2021.

Such a scenario would mean that Abe, who served previously as prime minister from 2006 to 2007, would be remembered as the longest-serving leader in modern Japan, topping Taro Katsura, who held the office for a combined 2,886 days before dying in 1913.

But Sunday’s trouncing has “clouded the prospect of a third term,” Narita said.

It also places a big question mark over Abe’s recently announced timeline for revising the pacifist Constitution by 2020. His goal is to alter war-renouncing Article 9 to make explicit the status of the Self-Defense Forces.

Abe’s sudden mention of a deadline and the unconventional ways in which he said he would go about legalizing the SDF caused murmurs of discord among his rivals in the LDP, such as Shigeru Ishiba, who argued the prime minister’s methods were riding roughshod over his party’s previous internal policies.

When he was riding high, Abe couldn’t have cared less about these voices of discontent, but now “it’s not clear whether he can push ahead with his agenda, given an expected rise in momentum within the LDP for questioning his power,” Narita said.

Speculation is also rife that Abe will attempt to engineer a make-or-break image makeover, perhaps starting this month, by overhauling his scandal-tainted Cabinet and ousting Defense Minister Tomomi Inada, who violated her obligation to stay politically neutral by hinting that the SDF backed an LDP-backed candidate.

But a failure to check the background of her replacement or other new faces would risk the possibility of further scandals, which would be a recipe for disaster, Narita said.

Katsuyuki Yakushiji, a professor of political science at Toyo University, said the metropolitan assembly election not only debunked the notion of Abe’s invincibility, but also exposed an uncomfortable truth.

With the main opposition Democratic Party so hopelessly unpopular, Abe had faced no real enemy until Sunday’s election. But when confronted with a powerful alternative in the person of Koike, the fragility of his popularity was made painfully clear.

“When there is no real alternative to the LDP, you either refuse to vote or begrudgingly vote for the LDP,” Yakushiji said. “Abe mistook these votes as positive support for his party, which emboldened him into going ahead with all these controversial policies, such as revising the Constitution.”

All eyes are now on whether Tomin First’s overwhelming victory will facilitate a foray by Koike into national politics — and ultimately her widely rumored bid to become Japan’s first female prime minister.

Asked Monday about the possibility of returning to national politics, Koike ruled out the idea. Observers, too, are playing it down, saying the fact she is Tokyo governor means she is unlikely to flirt with national politics anytime soon. Doing so, they say, would be taken as a betrayal of Tokyo’s residents that would send her popularity plummeting.

“She is not stupid enough to do that,” Yakushiji said.

Reports say some prominent lawmakers, including Koike protege Masaru Wakasa and ex-DP member Akihisa Nagashima, are thinking about founding a new “pro-Koike” party at the national level.

But Koike’s absence from such a party, coupled with the fact that no national election is scheduled in the immediate future, means the impact of such a move would likely be limited, Narita said.

“She’s a very ambitious person, so I’m sure she wants to be prime minister someday,” he said. “But at the same time, she’s also determined not to miss the honor of presiding over Tokyo as it hosts the Olympics and Paralympics in 2020. Her current term is set to expire right before the games, so she has to run for a second term as governor. There are still years to go.”