Local media and analysts have said that the victories for Hashimoto's Osaka Restoration Association could see the 46-year-old withdraw his pledge to retire, and that Prime Minister Shinzo Abe is likely to woo him to support key policies, including changes to the country's pacifist constitution.
Hashimoto experienced a setback in May, when the Osaka electorate rejected his plan to reform the city and help it to emerge from Tokyo's shadow.
However, Ichiro Matsui, 51, who was re-elected for his second four-year term as Osaka governor, and former lawmaker Hirofumi Yoshimura, 40, who won the Osaka mayoral election to succeed Hashimoto, have promised to take over Hashimoto's reform policies.
The two politicians have shared ideas on amending the constitution imposed by the United States after World War II.
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The Mainichi Shimbun newspaper said the double victory would allow Hashimoto to regain his political influence amid speculation that he would run in national elections himself in a bid to expand his party's presence in parliament and re-launch the Osaka reform plan.
His plan is aimed at slashing waste and strengthening the city's brand at home and abroad and would see Osaka's 24 separate wards merged into five special districts.
His plain-speaking and ability to ride out controversy won him enough admirers to be mentioned as a possible future prime minister.
But he once prompted outrage by suggesting battle-stressed soldiers needed the services of sex slaves from Korea, China, the Philippines and elsewhere who were forcibly drafted into Japanese brothels during World War II.