* Ruling parties lose first poll after historic security policy shift
* Rare win for weakened opposition Democratic Party
* Anti-nuclear sentiment likely affected outcome (Adds comment by Prime Minister Shinzo Abe)
By Linda Sieg
TOKYO, July 14 (Reuters) - A candidate backed by Japan's ruling coalition lost a race for a governorship on Sunday in an apparent backlash against Prime Minister Shinzo Abe's decision to end a security policy that has kept the military from fighting abroad since 1945.
The election in the western prefecture of Shiga was the first high-profile poll since Abe's cabinet adopted a resolution ending the ban on exercising "collective self-defence", or aiding a friendly country under attack - the most dramatic change in Japanese security policy in decades.
Abe has argued the change is needed to cope with a tough security environment, but the move has stirred angst among many voters wary of entanglement in foreign wars and worried that Japan's post-war pacifist constitution is being gutted.
Takashi Koyari, who ran with the backing of Abe's Liberal Democratic Party and its junior coalition partner, was defeated by former opposition Democratic party lawmaker Taizo Mikazuki, according to the prefecture's final vote count.
The loss sends a warning signal to Abe's administration, whose voter support has dropped below 50 percent in public opinion surveys after the July 1 shift in security policy.
"This confirms that this was not a popular idea," said Sophia University professor Koichi Nakano. "The LDP will continue to have to worry about more defeats to follow."
Three more gubernatorial races are set for later this year, to be followed by a slew of local polls nationwide next April. No general election is mandated until 2016.
The result was also a rare victory for the opposition. The Democratic Party, demoralised after losing to the LDP in the 2012 election that returned Abe to power for a rare second term, steered clear of formally endorsing Mikazuki but sent top executives to campaign on his behalf.
Abe, in an interview published on Monday, sought to soothe voter concerns over collective self-defence, while acknowledging more explanations were needed to win full support.
"Although we speak of the right of collective self-defence, this ... does not mean we will fight for other countries," he told the Mainichi newspaper. "The conditions are very strict."
Voters were also likely swayed by concern over the safety of nuclear power more than three years after the March 2011 Fukushima nuclear disaster, the world's worst since Chernobyl.
Shiga is adjacent to Fukui prefecture, home to a number of nuclear plants. Abe's government is keen to restart operations at some of the nation's 48 reactors, all of which are now offline after the Fukushima crisis, to reduce the cost of electricity and imported fuel.
Candidate Mikazuki had urged a gradual exit from reliance on nuclear power and called for Shiga to have a greater say in whether reactors in nearby Fukui are restarted.
Abe's government, with one eye on the coming local elections, has begun stressing the need to ensure that his "Abenomics" recipe for reviving the economy through easy monetary policy, spending, and structural reform, helps not only big cities and corporations but outlying regions as well.
(Reporting by Linda Sieg; Editing by Larry King and Michael Perry)