By Kiyoshi Takenaka and Linda Sieg
TOKYO (Reuters) - The son of a U.S. Marine and Japanese mother won an election for governor of Japan's Okinawa island on Sunday on an anti-U.S military presence platform, defeating a candidate backed by the ruling bloc in a blow to Prime Minister Shinzo Abe.
Denny Tamaki, who was backed by opposition parties, soundly defeated Atsushi Sakima, who ran with the support of Abe's Liberal Democratic Party and its junior coalition partner, the Komeito, official results showed.
Public broadcaster NHK showed a jubilant Tamaki and his supporters cheering and dancing after news of his victory.
Sakima's defeat is a blow for Abe, as the Okinawa vote was the first high-profile election after the premier won a Sept. 20 LDP leadership vote, putting himself on track to become Japan's longest-serving premier.
Senior politicians from the ruling bloc had visited Okinawa over the past few weeks to throw their weight behind Sakima, underlining the significance of the outcome of the election.
"The fact that the LDP and (junior coalition partner) Komeito ... resorted to all means available and still lost against a united opposition is a serious blow," said Sophia University political science professor Koichi Nakano.
Japan's national level opposition is fragmented, a factor that has helped the LDP win national elections since Abe took office in 2012.
A former radio personality who went into local politics and was elected to parliament in 2009, Tamaki, 58, opposes the planned relocation of the U.S. Marines' Futenma air base within Okinawa. His stance mirrored that of his predecessor, Takeshi Onaga, who died of pancreatic cancer while in office.
Tokyo's central government and Okinawa authorities have long been at loggerheads over the plan to move the air base.
A U.S.-Japan agreement calls for moving the base, which is surrounded by schools, hospitals and shops, to a less populated area, called Henoko, on the northern part of Okinawa.
But many Okinawa residents, indignant at what they see as an unfair burden for supporting the bulk of U.S military forces in Japan, want the base off the island altogether.
Tamaki's victory follows soul searching in Japan about what it means to be Japanese after Naomi Osaka, 20, won the U.S. Open tennis title, the first Grand Slam win by a Japan-born tennis player. Osaka is the daughter of a Haitian-born father and Japanese mother and has dual Japanese and U.S. citizenship.
Sakima had focused on economic policies to spur Okinawa's growth for Japan's poorest prefecture during his campaign.
Delays in relocating the U.S. base have been an irritant in Japan's relations with ally the United States at a time when Tokyo, like other countries, is faced with an increasingly assertive China.
(Writing by Kiyoshi Takenaka and Linda Sieg; Editing by Jane Merriman and Neil Fullick)