IN DIFFERENT ways the United States is serving notice on both Palestinians and Israelis that this country's participation in efforts to build a peace between them cannot be taken for granted. This is a fairly momentous move, for there is a risk in making such an assertion that the parties will drift further into tension and hostility. But there is also a risk that without the setting of such conditions, the United States will waste its diplomatic capital and regional deterioration will spin on anyway.

To the PLO, Washington continues to insist that it condemn and discipline the faction that launched a disgusting terrorist raid on Israel two weeks ago. To do so would merely be in accordance with the PLO's pledge when it opened a dialogue with the United States late in 1988, so it is hardly a lot to ask. Rather it is the minimum. An American wink or any other kind of acquiescence would undercut the struggle against terrorism, tarnish American credibility across the Middle East -- and no doubt stir Congress to act on its own to break the American-PLO link. PLO chairman Yasser Arafat is dancing around: he is reluctant not only to lose the PLO's hard-won Washington connection but also to make another payment for it. Equivocation is the traditional unifier of the Palestinian national movement -- and the curse of progress toward a Palestinian state. The PLO cannot expect others to respect their obligations to Palestinians if it does not respect its own.

To the new Israeli government, Secretary of State James Baker has now said that unless it makes the compromises necessary to open Israeli-Palestinian talks, the United States will back off from Mideast diplomacy. He delivered this message in a sharp tone that itself became an item in the news. Still, the tone was mellow and the message moderate next to the policies announced by Yitzhak Shamir. His Likud party has changed partners, having released Labor, which had pulled Likud somewhat toward the center, and having embraced alliance with conservative parties that are pulling it far to the right. Under the new Likud government, talks with Palestinians move from almost-within-grasp to out-of-reach. In these circumstances, Mr. Baker's response was understandable.

It would be unfortunate to see the American diplomatic door closing on the PLO, if only because the PLO is an important player in a region important to the United States. It would be doubly unfortunate to see an American diplomatic door of another sort closing on Israel, suspending one of the chief instruments -- a shared Mideast policy -- for supporting the United States' deep friendship and abiding concern for the Jewish state.