JERUSALEM — FEARS for the future of Middle East peace and concern that Soviet Jews might now be trapped in the Soviet Union haunted Jerusalem yesterday, in the wake of Soviet President Mikhail Gorbachev's dramatic overthrow.Israeli officials took solace in the new Kremlin chiefs' pledge that Moscow would stand by its foreign commitments, but were worried by the ouster. "We pray that the rapprochement between the two blocs, the hope that difficult days are behind us ... will not be cut off," said Foreign Minister David Levy. That prayer reflected a clear awareness here, two months away from a planned Mideast peace conference sponsored by Washington and Moscow, that the future of the Middle East hinges largely on whether the two superpowers retreat into their previous cold war postures, or whether they maintain some cooperation. "A basic part of Gorbachev's philosophy was that the superpowers should work together to solve regional problems," says Stefani Hoffman of the Mayroock Institute for Soviet and Eastern European Research. "It will be important to see what sort of cooperation there is." Her colleague Galia Golan, another Soviet affairs expert, is pessimistic. "I think [the new Soviet rulers] will opt for much stronger competition with the United States," she warns. "These people are very definitely interested in a restoration of Soviet power around the world and in the Middle East." Nor are the hard-liners now in charge in Moscow seen as at all sympathetic to Israel, and observers here doubt whether they will continue former President Gorbachev's moves to restore full diplomatic relations with the Jewish state. Such relations, however, are a condition Israel has set before Moscow can co-sponsor the regional peace conference proposed for October. Moscow's presence at that conference, strongly urged by Syria and the Palestinians, is now uncertain. Even if Moscow does co-chair the meeting, possible changes in the new government's attitude to the Middle East could be crucial. "It makes a big difference whether you go in as a neutral party or as a champion of the Arabs," says Ms. Golan. Mr. Levy also says he hopes that "in the Soviet Union, whoever it will be, understands the significance of leaving the gates of the Soviet Union open to the Jewish nation." VER a quarter of a million Jews have left the Soviet Union for Israel over the past 18 months, taking advantage of the freedom to emigrate granted by Gorbachev. Another 60,000 have all their papers ready to leave, according to Uri Gordon, head of immigration for the quasi-governmental Jewish Agency that brings Jews to Israel. "It could very well be that in a short period of time a large number of Jews ... will arrive here," he told Israel Radio yesterday. It was unclear whether the Soviet borders were open as usual yesterday, and whether the new authorities would maintain the previous government's policy on emigration. Jewish Agency officials, however, were in emergency session on yesterday afternoon, preparing for a sudden flood of arrivals. "A feeling of uncertainty about the future has always been the strongest motivation for the emigrants, and that will only be stronger now," says Dr. Hoffman. "So long as it is possible, we will see more coming out." Although Golan says she fears that "Jewish emigration is not a policy the new lot will support," Soviet Jewish activists pin their hopes on Moscow's deciding that it still must court the West for economic reasons. "I hope very much that the new leaders will understand that in the end they will have to turn to the West, and that the only card they have in their hands is the emigration of the Jews," says former dissident Natan Sharansky. "So in the long run at least, immigration from the Soviet Union would not be harmed."