IN a journey of thousands of miles, the last step is proving to be the most difficult. After five trips to the Middle East, United States Secretary of State James Baker III has come within inches of convincing Israel and its Arab neighbors to seize the moment provided by the end of the Gulf war to negotiate face to face. One hurdle remains, however, and it is the highest one of all: the composition of a Palestinian delegation to bargain with Israel over the future of the Israeli-occupied West Bank and Gaza Strip. Mr. Baker will travel to the region today for talks with Israeli leaders. Soviet Foreign Minister Alexander Bessmertnykh promised a later trip to extend diplomatic recognition to Israel if the Jewish state agrees to enter peace talks. "This historic opportunity must not be lost," Mr. Bush said in Moscow yesterday. To stave off deadlock, Baker is now said to be providing quiet assurances to Israel even as he embarks - according to Israeli sources at press time - on a sixth trip to the region. "We ought to wrap it up," Baker told reporters in Moscow this week. A US plan calls for a one-time peace conference leading to direct negotiations between Israel and its four Arab neighbors, Syria, Lebanon, Jordan, and West Bank and Gaza Palestinians. After the direct talks began, Israel and Arab countries other than the aforementioned would enter discussions on various regional issues, including arms control and the worsening water crisis in the Middle East. The main sticking point now is Mr. Shamir's refusal to agree to any conference attended by representatives of the Palestine Liberation Organization (PLO) or by Palestinians from Arab East Jerusalem, which Israel insists is permanently part of Israel. Its negotiating position weakened because of its support for Iraq in the Gulf war, the PLO is likely to settle for the indirect role of approving a list of non-PLO Palestinians to attend peace talks. But Palestinians have demured on a proposed US compromise under which East Jerusalem Palestinians would be excluded until eventual talks begin on the final status of the West Bank and Gaza. According to news reports, the US is providing renewed assurances to Israel that Arabs from East Jerusalem will be excluded. As a compensatory gesture to Palestinians, it is also weighing a first-time declaration of support for Palestinian self-determination. The Bush administration will also reassure Palestinians that it shares their interpretation of UN "land for peace" resolutions as including the West Bank and Gaza. Israel's Likud government says the resolutions apply only to the Sinai Peninsula, which was relinquished to Egypt under the Camp David Treaty in 1979. After consulting with the PLO, Palestinians are expected to ask Jordan to form a joint delegation. It would likely be headed by Jordanian Prime Minister Tahir Masri, a Palestinian from a prominent West Bank family. "Tahir Masri is enough of a Jordanian for Israel and enough of a Palestinian for Palestinians," comments one Israeli journalist in Washington. Most observers agree that the conference could founder almost immediately over the land-for-peace issue. For its part, the Bush administration is banking on the possibility that the talks could be a confidence-building process that, in time, could lower the resistance of both sides to making concessions needed for peace. If negotiations do begin, the temptation for Israel will be to attempt a separate peace with Syria by making concessions on the Golan Heights, which Israel captured from Syria and annexed in 1981. That would ease pressure to cede territory in the West Bank which is more important to Israel religiously and historically. If the negotiations falter, the consequences for regional stability could be grave, most Middle East observers agree. "There will be a continuation of upheavals in the territories and a continuation of resentment and bitterness throughout the Arab world," says one Arab diplomat in Washington. If peace talks are finally convened, Shamir may be seen as an unintended victim of his own success. It was he who started the diplomatic ball rolling toward peace when he put a peace plan on the table in May 1989, fully expecting, in the view of many Israelis, that the Arab states and Palestinians would reject it. Even though peace efforts collapsed last year because of opposition from right-wing parties in Israel, the US persisted in its efforts to bring the parties together. When Syria finally agreed to the compromise terms offered by Baker two weeks ago, Shamir was left as the only obstacle to the start of peace talks.