Mayor Osami Takeyama’s victory Sunday in the Sakai mayoral election in Osaka Prefecture has cast a shadow over the political future of brash Nippon Ishin co-leader Toru Hashimoto, the mayor of Osaka.

Takeyama, a 63-year-old independent, defeated former Sakai Assemblyman Katsutoshi Nishibayashi, his sole challenger, with the backing of the Democratic Party of Japan and the ruling Liberal Democratic Party, early returns showed.

Nishibayashi, 43, was backed by Nippon Ishin no Kai (Japan Restoration Party).

The main issue in the election was whether to let prefecture absorb the major cities by turning them into wards, as in Tokyo. Sakai would have been one of those cities.

One of Hashimoto’s main goals is to integrate the city of Osaka with the prefecture, giving it the same administrative structure and status as Tokyo.

During his campaign, Takeyama 63, argued that integration would only “steal Sakai’s financial resources and authority.”

Nishibayashi claimed that Sakai would be “left behind” in terms of economic development unless it was part of the larger entity.

Takeyama was elected to his first term four years ago with Hashimoto’s backing when Hashimoto was the governor of Osaka Prefecture. He later butted heads with Hashimoto over the integration plan, making it clear that Sakai would not take part in the merger.

Despite Sunday’s defeat, however, Hashimoto denied at a news conference in Sakai that he would resign as Nippon Ishin’s co-leader.

The Sakai election and Nippon Ishin are “separate matters,” he said. “I must fulfill my responsibilities.”

He also said his goal of creating a larger Osaka would not be deterred.

Even so, the election defeat will inevitably affect his already weakened clout in national politics, observers said.

Indeed, a Kyodo exit poll showed that more than 66 percent of so-called swing voters in the election sided with the incumbent — a sign that Hashimoto is fading fast.

Hashimoto’s meteoric rise to local stardom, and then national politics, owes much to the support of this segment of the voting populace. But Sunday’s poll suggests that Nippon Ishin’s repeated conservative forays, as well as inflammatory remarks made by its own leaders, have disillusioned many of its supporters.

Earlier this year, Hashimoto enraged people at home and abroad by saying he believes Japan’s wartime brothels, where thousands of women and girls were forced into sexual servitude, was “necessary to maintain discipline” in the Imperial Japanese military.

In addition, feisty co-leader Shintaro Ishihara, the one who came up with the plan to purchase the Senkaku Islands from their Japanese owner to further assert Japan’s claim to sovereignty, only resulted in bringing ties with China to their lowest in years, and dragging down the economy.