THE new opposition party Shinshinto (New Frontier Party) does not immediately threaten the ruling coalition of Prime Minister Tomiichi Murayama, since it is not as large as originally expected. But the future of the Murayama Government is far from assured in the wake of Shinshinto's formation simply because no single political party comes close to having a parliamentary majority in its own right. Additionally, there is unrest in the Socialist party which could topple the present coalition Government. These conclusions emerge from the latest headcount of party loyalties in the Japanese Diet in the wake of nine smaller parties or factions ostensibly sinking their differences under the Shinshinto banner. Shinshinto, inaugurated last Saturday amidst much fanfare, is smaller than expected, holding 179 seats in the lower House of Representatives. Earlier it was expected to attract at least 190 lower house members, a figure closer to the 200 members belonging to the largest single parliamentary party, that of the long-ruling Liberal Democratic Party (LDP). In the House of Representatives, Shinshinto is the second largest party, being more than twice the size of Mr Murayama's Socialist Party which has 72 seats. But in the upper House of Councillors the Socialists are almost double the size of Shinshinto, holding 68 seats to the new party's 36. The LDP has 95 upper house seats. Shinshinto's upper house weakness is a result of a split in the former Clean Government Party, or Komeito. Prior to the new party's formation, Komeito split in two with 12 of its upper house members refusing to join Shinshinto until the middle of next year. On the surface, Mr Murayama's Cabinet has a clear majority in both houses. A small party called Shinto Sakigake (New Pioneer Party), with 21 lower house members, gives the government coalition a total of 293 out of 511 seats. But, as Japanese politics has illustrated recently, a parliamentary situation in which no single party has a majority can easily be a recipe for instability. Right now, Mr Murayama's Socialist party clearly feels pressured by the looming presence of two basically conservative parties - the LDP with a total of 295 members in both houses, and Shinshinto with 215 members. An unknown, but probably sizeable, proportion of the Socialists' 140 members are believed to favour the formation of a new third party which is more liberal and progressive than either the LDP or Shinshinto. Only 37 Socialist defections, half its lower house strength, will bring down the Government. Moves towards the third party, already underway, could gravely weaken Mr Murayama's ruling coalition and force a new House of Representatives election next year, even though the present house could continue until 1997. One leading Socialist who favours a new party has put off immediate action on the project at Mr Murayama's request - but whether this solves the longer-term problem facing the Prime Minister remains in doubt. Socialist Secretary-General Wataru Kubo told the Kyodo news agency this week that he would not form a 'liberal, democratic' party before the next parliament is convened at the end of January. Half of the upper House of Councillors, and most local government officials are due to be elected next year anyway and this also increases pressure for a 1995 lower house election.