Official campaigning got under way Thursday for the Sept. 30 Okinawa gubernatorial election, with the controversial plan to relocate a key U.S. military base within the prefecture topping the agenda.

Atsushi Sakima, 54, a former mayor of Ginowan, backed by Japan's ruling parties, and former opposition lawmaker Denny Tamaki, 58, are the main contenders for the post left vacant by the death last month of Takeshi Onaga, who was a staunch opponent of the plan.

The outcome of the race could affect the future of the plan to move U.S. Marine Corps Air Station Futenma from the crowded residential area of Ginowan to the less populated coastal district of Henoko in Nago, both on the island of Okinawa.