* Regional security threats limit options
* Democrats focused on domestic priorities
* Party seen muddling through until 2010
By Paul Eckert, Asia Correspondent
WASHINGTON, Aug 26 (Reuters) - Japan's populist opposition Democratic Party, forecast to win Sunday's election, will likely bring confusion rather than dramatic foreign policy changes to the United States' main Asian ally.
Polls show the Democratic Party of Japan (DPJ) could win by a landslide, ending more than 50 years of rule by conservatives who kept Japan in lock-step with Washington on security policy in return for the shelter of its "nuclear umbrella".
The left-of-center DPJ, an amalgam of conservative and former Socialist ideological factions, had advocated standing up to the United States and moving closer to Asia. But it has edged toward the status quo as the Aug. 30 vote draws near.
Japan lives in a dangerous neighborhood, which means the DPJ would probably only tinker with the long-ruling Liberal Democratic Party's (LDP) policy of making the U.S.-Japan alliance the core of Tokyo's diplomatic and security policies.
North Korea in May conducted its second nuclear test since 2006, and has launched several missiles over Japan and into the Pacific. China's rapidly growing military power is another worry for Japan, whose defense spending has been sliding for seven years.
MUDDY UNTIL 2010?
The DPJ is running mainly against LDP domestic policies. It has pledged to redistribute income, spend more on households, cut waste and wrest control of policy from bureaucrats.
"Foreign policy is going to be a second- or third-tier issue for them, barring a crisis, for at least a year," said Michael Auslin of the American Enterprise Institute, a conservative Washington think tank.
"We'll get a much clearer picture in 2010 of what the foreign policy priorities of the DPJ are," he said.
U.S. Japan-watchers see a year in which the DPJ focuses on its ambitious domestic agenda, while fudging on foreign policy in order to keep the support of the Socialist Party in the upper house of parliament which faces elections in 2010.
"They don't have a popular mandate to abandon the U.S.-Japan alliance or challenge the Obama administration, so for the next year their foreign policy will be confusing," said Michael Green, a former Bush administration White House official.
Still, the DPJ will try to placate its leftist coalition partners, said Green, now Japan scholar at the Center for Strategic and International Studies in Washington.
"You may have a year of frustrating U.S.-Japan alliance relations, difficulty over bases, vague policies, an inability to mobilize Japanese efforts in Afghanistan or elsewhere."
HOLDING PATTERN
DPJ officials have questioned the utility of the 47,000 U.S. troops based in Japan, attacked a plan to move 8,000 U.S. Marines from the southern island of Okinawa to Guam, and threatened to stop Japan's refueling mission in support of U.S.-led military operations in Afghanistan.
But most of those stances have been set aside to appeal to Japanese voters and avoid making waves in Washington.
"As the DPJ becomes responsible for government decisions, I don't think they are going to be taking as drastic actions that some people forecast," said W. Stephen Piper, head of the consultancy Piper Pacific International.
"It's going to be a holding pattern. Bold talk? Yes. Dramatic departures? No," he said.
Some experts caution the Obama administration not to expect too much of the new government expected to formed by Democratic Party leader Yukio Hatoyama. The 62-year-old grandson of a former prime minister could make his international debut at the G20 summit in Pittsburgh in late September.
The United States should show patience and not lean too heavily on the new government, Yoichiro Sato of the Asia-Pacific Center for Security Studies advised in an essay for the Hawaii-based Pacific Forum/CSIS think tank. (Editing by Alan Elsner)