* Ex-defence minister, former PM Abe, in 2nd round vote
* New LDP leader likely to become next prime minister
* Contenders promise tougher stance in dealing with Beijing (Updates with first-round vote result)
A former Japanese defence minister and an ex-premier will square off on Wednesday in a run-off vote to become leader of the main opposition party and possibly the next prime minister who will have to manage ties with China badly frayed over a territorial dispute.
Shigeru Ishiba, 55, a conservative security expert, and hawkish former prime minister Shinzo Abe, 58, will face off in a final party ballot due at around 2:30 p.m. (0530 GMT) after defeating Liberal Democratic Part (LDP) number two Nobuteru Ishihara and two other contenders.
The top contenders in the race to lead the conservative LDP have all struck hawkish tones as a long-simmering row with China flared this month after Prime Minister Yoshihiko Noda's government bought some disputed islets in the Eas t China Sea fro m their private Japanese owner.
The LDP, long a broad umbrella-party home to both security hawks and doves, has tilted to the right in recent years. Noda also hails from the conservative wing of his diverse party.
Ishiba says Japan's pacifist constitution should be revised to ease restrictions on military action and called for Japan to have its own Marine Corps.
Ex-prime minister Abe, who also advocates revision of a U.S.-drafted constitution, wants to replace a historic 1995 statement by then-prime minister Tomiichi Murayama, apologising for suffering caused by Japan's wartime aggression. That is hardly likely to win friends among Asian neighbours.
Opinion polls suggest that the LDP, ousted in 2009 after more than 50 years of almost non-stop rule, will come first in a lower house election expected within months, but will need a coalition partner.
A tie-up with a party led by populist Osaka Mayor Toru Hashimoto, which critics say is tapping simmering nationalist sentiment, could drag a new coalition to the right, making it harder to manage already rocky ties with China and South Korea.
Abe, who quit due to ill-health in 2007, is eyeing a possible alliance with Hashimoto. Ishiba could also be a partner for the 43-year-old Osaka mayor.
Whoever takes charge of the world's third-largest economy after the general election, expected as early as November, will face various unfinished business and deep-rooted problems dogging the economy.
The rebuilding of northeast Japan after a March 2011 earthquake and tsunami is far from complete and the full decommissioning of the crippled Fukushima nuclear plant will take decades.
Any new government will have to grapple with a revamp of energy policy amid deep public worry about nuclear safety.
On top of that, a plan to double the sales tax that Noda managed to push through an opposition-controlled upper house by promising early elections is seen as just a first step towards reining in Japan's ballooning public debt.