Japan's Prime Minister Yukio Hatoyama has said the result of a weekend mayoral poll could fuel a major rethink about US military bases in Japan.
Residents of the Japanese city of Nago, on Okinawa, chose a candidate opposed to the hosting of an American air base.
The Futenma base was originally scheduled to move to Nago from a more crowded part of Okinawa.
Talk of moving the base out of Japan altogether has threatened the long-standing US-Japan security alliance.
Mr Hatoyama said the results of Sunday's election reflected the will of the people, and that Japan will continue to re-examine its commitment to relocate the air base.
"The country will start from scratch on this issue and take responsibility to reach a conclusion by the end of May," he told reporters.
'Post-war strategy defeated'
The new mayor of Nago, Susumu Inamine, had campaigned against any expansion of US military presence in the area. He beat the incumbent, Yoshikazu Shimabukuro, who supported the base, by winning 52.3% of the vote.
Mr Inamine, an independent, ran with the support of Mr Hatoyama's ruling Democratic Party.
Correspondents said his victory will make it increasingly difficult for the prime minister to resist pressure to shelve the deal.
"I fought this campaign vowing to resist the base. I intend to keep that promise as we move forward," Mr Inamine said.
National daily newspaper Asahi said in a front-page editorial on Monday: "It wasn't just Shimabukuro that was defeated in the election. The biggest loser was Japan's post-war military base strategy."
Correspondents said the prime minister's comments highlighted the difficulties involved in fulfilling an agreement with the US to relocate the base against strong local opposition.
Japan signed a deal with the US four years ago that was part of a broader realignment of American troops.
A key part of the plan was to relocate the Futenma air base, home to about 2,000 Marines, to the smaller city of Nago.
Okinawa is home to most of the 47,000 American troops based in Japan.