SINCE it lost control of parliament's upper house in 1989, Japan's ruling Liberal Democratic Party has agonized over how to pass contentious legislation, such as a bill to deploy troops overseas.The LDP may have found its answer in Makato Tanabe, who was elected July 23 as the new chairman of the leading opposition party, the Japan Socialist Party (JSP). "What's important for Japan is that we cooperate [with the LDP]," he said in a recent interview. With close personal links to LDP chieftains, Mr. Tanabe paints himself as both conservative and flexible, in sharp contrast to his hard-line leftist predecessor, Takako Doi. It was Ms. Doi, the first woman to head a Japanese party, who championed the 1989 victory, only to make mistakes that cost the party dearly in two elections this year. Doi resigned last month, leaving the party in direction-less internal strife. Trying not to let its popularity fade away even more, the party recently softened its English name by calling itself the Social Democratic Party of Japan, although its Japanese name still translates into the Japan Socialist Party. Since the LDP has been in power for 35 years and the Socialist Party has been so weak, Japan's democracy has been characterized as a one-and-a-half party system. "The JSP has fallen badly," says political science professor Takeshi Kono of Kyorin University. "Japanese people no longer vote just for conservative or reformist, but on the policy issues at the moment." A poll in June by the Asahi newspaper showed the LDP at its highest approval rating since its formation in 1955. The socialists were particularly hurt during the Gulf war. When public opinion finally moved to supporting the war, the party was marooned by its antimilitary stand, which holds that Japan's "self-defense force" is unconstitutional. A leader of the JSP's right-wing faction, Tanabe may try to ease his party's opposition to LDP proposals to revamp Japan's political structure and to allow the military to serve in United Nations forces. But the party itself, which still includes antimilitary and anti-American leftists, may have a difficult time forsaking its well-worn role as a spoiler to almost all LDP proposals. "The party is on the decline because it has been unable to work with the unions, its strongest supporters, as well as with the government," says political science professor Nobuo Tomita of Meiji University. "The only way for it to survive is to link up with civic activist groups." Tanabe, a lower house member for 31 years, showed how much he can cooperate with the LDP by traveling to North Korea last fall with an LDP kingpin, Shin Kanemaru, in the first big breakthrough in relations between the two estranged countries. The socialists have kept close ties with North Korea's communist party. The socialists have also been isolated by the LDP's ability to woo over two smaller opposition parties recently, the Buddhist-backed Komeito and the Democratic Socialist Party, in passing key legislation.