THE leaders of seven Japanese opposition parties are due to finally come together today to announce their joint plan for ending 38 years of continuous rule by the Liberal Democratic Party (LDP). The end of LDP dominance became almost certain yesterday when two opposition parties with the power to still give either the opposition or the LDP a working majority, finally rejected LDP efforts to secure their support and cast in their lot with the opposition. As they did so, the LDP itself split along generational and policy lines as only two candidates entered the race for the party presidency, a contest which became necessary following the resignation of caretaker Prime Minister Kiichi Miyazawa. The first change of government brought about by Japanese voters under the 1947 Constitution became possible in the wake of the July 18 general election when the LDP, in power continuously since 1955, lost 42 seats out of the 275 won in the last election in 1990. Five opposition parties had discussed the possibility of a coalition to replace the LDP before the election, on the initiative of the largest party formed by LDP defectors, Shinseito (Japan Renewal Party). The other four parties covered the political spectrum from the left-wing Japanese Socialists to the left-of-centre Shaminren (Social Democratic Union) to the centrist Democratic Socialist Party (DSP) and the Komeito (Clean Government party). Both the LDP and the opposition coalition of five parties needed the support of the two other new parties formed from LDP defectors - Nihon Shinto (Japan New Party) and the Sakigake (New Forerunner Party). After the election, Nihon Shinto and Sakigake quickly formed a parliamentary alliance with their 48 seats. The crucial move yesterday came when the Nihon Shinto-Sakigake alliance met LDP officials and rejected the LDP's bid for their support. Those 48 seats would have given the LDP not merely a majority in the House of Representatives (256 seats) but a working majority of 271 seats, enabling the Government to control all Diet committees. The suspense over yesterday's meeting was greater because last Friday the Nihon Shinto-Sakigake alliance had already indicated its preference for joining the opposition alliance but appeared to backtrack earlier this week when announcing that it would meet LDP representatives again. Suspicion increased that Nihon Shinto and Sakigake were putting power before principles. Anxious to win them over, the LDP announced that it would accept the Nihon Shinto-Sakigake insistence on electoral reform consisting of a new House of Representatives which was half elected in single-seat constituencies, and half-elected by proportionalrepresentation. But the LDP failed to win over the alliance with its promises. ''The relationship between our two parties and the LDP has ended,'' said Sakigake leader and former prefectural governor Masayoshi Takemura. ''The LDP proposal has no substance at all,'' former governor Morihiro Hosokawa, the founder and leader of Nihon Shinto, said after meeting Hiroshi Mitsuzuka, one of the LDP's top officials. ''From now on, it will be the seven opposition parties against the LDP,'' Mr Takemura said, leading to major headlines in Tokyo's evening papers announcing the imminent end of LDP rule. The LDP's reduced prospects for retaining power have been reflected in the withdrawal of several candidates from the leadership contest due to be decided tomorrow. The only two now left are former foreign minister Michio Watanabe, a veteran who resigned earlier this year due to protracted illness, and the current Chief Cabinet Secretary, Yohei Kono, 56, who ironically himself once broke away from the LDP to form a new political party called the New Liberal Club. Apart from the 14 years separating them, the two candidates view political reform in a different light. Mr Watanabe is clearly reluctant to pursue it, whereas Mr Kono supports change in the political and electoral system. According to the unwritten rules of LDP factional politics, Mr Watanabe should win. He leads a sizeable faction himself, whereas Mr Kono does not. Additionally, Mr Kono belongs to the Miyazawa faction, and LDP stalwarts will feel another faction should have its turn occupying the presidency. But there may be one faction less taking part in the LDP election. Yesterday the leader of a small faction of 12 members, former top LDP official Mutsuki Kato and two of his colleagues resigned from the LDP, sparking fears in the party that there will be more defections to the opposition, especially as it now appears capable of taking power. However, once the LDP's new leader is chosen, the party can be expected to make one more effort to break up the seven-member opposition coalition. Already the day when the new prime minister - and therefore the new government - will be chosen by a vote of the House of Representatives appears to have been put back from August 2 to August 6, which gives the LDP four more days to try to find a way toretain power.