POLITICAL necessity is slowly driving the Middle East's longtime adversaries toward their appointed meeting in October. No one wants to be branded an obstacle to peace, and all are looking for assurances from the United States that their interests will be protected.Israel has obvious reasons to heed the call - issued jointly by Washington and Moscow - to come to the peace table this fall. A refusal would have dimmed prospects for US loan guarantees to help resettle Soviet Jewish immigrants. But that's only one, very immediate concern. Even as he uttered a conditional "yes" to the conference, Prime Minister Yitzhak Shamir was angling for guarantees that his fundamental concerns - Israeli security needs on the Golan Heights, for example, or continued sovereignty over a united Jerusalem - will be uppermost in the minds of President Bush and Secretary of State Baker. The Palestinians and Syrians are equally insistent on a US commitment to stick with the "land for peace" formula spelled out in United Nations Resolution 242. The United States, like the Arabs, has always understood the resolution to apply to all lands taken by Israel during the 1967 war - not just Sinai, as Mr. Shamir's government argues. These concerns are matters of substance whose resolution is out of sight, far down the negotiating trail. For now, the tough bargaining revolves around the structure of a conference - most pointedly, who will represent the Palestinians. Shamir demands the exclusion of Palestinians who live in East Jerusalem, who are Palestine Liberation Organization members, or who come from the Palestinian diaspora. Palestinians contend they should be able to choose who they want, free of American or Israeli constraints. That's hard to refute on philosophical grounds. But as a practical matter, it's not going to happen. Diplomats are seeking ways to finesse the Israeli conditions, perhaps with a Jerusalem-born Palestinian now living in Jordan. When it comes to PLO ties, Shamir's negotiators may just have to blink. The PLO is likely to have a strong influence on anyone chosen from the West Bank or Gaza. It's difficult to be optimistic about the proposed gathering in October. The issues, if they're even engaged, address deep-seated matters of national identity. On both the Israeli and Palestinian sides, hard-line elements would like to see the whole process thwarted. Still, there's value in simply getting the adversaries to sit across from each other and talk. It just might thaw the Middle East's long-frozen politics.