Nippon Ishin no Kai (Japan Restoration Party) co-leader and Osaka Mayor Toru Hashimoto was facing a critical battle. And he lost.

On Sunday, a candidate fielded by Nippon Ishin lost a local government election for the first time in Osaka Prefecture, the power base for both Hashimoto and his party.

The result could mean the end of Hashimoto’s main policy goal of merging key cities in Osaka Prefecture into a single administrative entity. Sunday’s winner, Sakai Mayor Osami Takeyama, is openly and staunchly opposed to the idea. Sakai is one of the cities that would be merged under Hashimoto’s grand plan, which aims to give Osaka the same administrative status as Tokyo.

The loss in the Sakai election also weakens Nippon Ishin’s clout at the national level, including the effort by some of its members to organize opposition lawmakers into a single political force to stand against the Liberal Democratic Party.

According to the Sakai election board, the 63-year-old Takeyama received 198,431 votes in his re-election bid, while former municipal assembly member Katsutoshi Nishibayashi, 43, got 140,569. Turnout was 50.69 percent, up 6.76 points from the last mayoral race four years ago.

In that election, Takeyama won his first term with Hashimoto’s backing. This was when Hashimoto was still governor of Osaka Prefecture. The two, however, ended up clashing over the city merger plan.

An analysis of media exit polls clearly illustrates that Hashimoto’s appeal to swing voters has diminished. The surveys show that a majority of unaffiliated voters — the key element in many elections across the country — no longer support Nippon Ishin, even in Osaka Prefecture.

“Many people say unaffiliated voters have deserted (Nippon Ishin). It’s true that now we have already become an ‘old existing party,’ ” Hashimoto told a news conference in Osaka on Sunday. “To be honest, I would say unaffiliated voters now support (Prime Minister Shinzo) Abe. It’s clear to everyone he has momentum.”

Hashimoto, a former lawyer and popular TV commentator, emerged as a rising political star by attracting swing voters frustrated with the old major parties.

But according to an exit poll by the Asahi Shimbun, 69 percent of swing voters in Sakai voted for Takeyama even though he was supported by major parties including the LDP and Democratic Party of Japan.

Hashimoto’s meteoric rise in politics owed much to the debating skills and colorful rhetoric he uses during TV appearances and election campaigns.He has long stressed that his party’s main support base should be formed by frustrated swing voters and not by a rigid campaign machine like the major parties.

“We don’t have money or political power. What we can rely on are the ‘floaty voices’ of the people,” Hashimoto told an audience in Osaka on June 24, 2012. “These ‘floaty voices’ of the people are scary. Sometimes they give us big support. And they could desert us in a mere second.”

Hashimoto is now seeing how scary those voices can be.

Swing voters have apparently turned on Nippon Ishin as it has grown more radical in its nationalistic leanings and as Hashimoto’s fresh public image as an administrative reformist loses its luster.

According to monthly national polls by NHK, Nippon Ishin’s support rate plummeted from 6.5 percent in January to only 2.2 percent in September.

Hashimoto and his party feared they could be fatally damaged by a loss in Sakai.

Following the July 21 Upper House election, Hashimoto offered to resign as party co-leader to take responsibility for Nippon Ishin’s poor results. At the time, Hashimoto said he wanted to concentrate on the Sakai mayoral election, rather than national politics, citing it as one of the reasons he should step down as co-leader.

Hahshimoto eventually retracted his resignation after other party executives convinced him to stay on.

On Sunday, Hashimoto said he won’t step down. One reason may be that Nippon Ishin doesn’t have a high-profile politician to replace him with.

“Quit? Why should I quit?” Hashimoto said during Sunday’s news conference when asked if he would resign to take responsibility for the defeat. “I need to win elections again.”