THE IRAN CRISIS, now almost a month old, is in what is almost certainly its most volatile phase. Today is the Shiite Moslem holy day of peak popular emotion. Tommorrow is the scheduled opening of the United Nations Security Council's first formal address of the crisis. On Sunday a referendum on Ayatollah Khomeini's proposed new Islamic constitution is to begin. Mexico's sudden decision not to renew the shah's visa adds yet another level of complication. A countdown of great tension and uncertainty is building.

We offer no predictions on how events may unfold, but perhaps it is useful to point up some of the basic considerations. Of these, the most disturbing is the nature of the public authority in Tehran. A power struggle is going on related to the proposed constitution -- that is, to the kind of country Iran is to become: completely closed in a particular Islamic tradition or at least somewhat open to the 20th century. The "closed" faction has just ousted a foreign minister representing the "open" faction. There is no denying that this throws additional doubt over how the regime will respond to the heavy and virtually unanimous international pressures to release the hostages.

President Carter, meanwhile, in his television appearance Wednesday evening made a presentation carefully crafted for his two very different audiences. To Americans he gave the necessary assurances of faithfulness to the hostages and to American credibility and honor. To Iranians he projected a stern insistence that the Iranian government is responsible for its captives, and a quiet warning of "the grave consequences which will result if harm comes to any" of them. Yet he left the ayatollah a way out, saying that the question of an international court or forum in which Iran could state its grievances is, though unprecedented, "a matter that can be pursued -- it should be pursued under international law . . . . [But] I don't think there's any forum that will listen to the Iranians make any sort of claim, justified or not, as long as they hold against their will and abuse the hostages, in complete contravention to every international law and every precept or every commitment or principle of humankind."

In brief, Jimmy Carter has done what he has had to do. And while there is a certain restiveness in some quarters on account of his patience and restraint, more than on any other occasion in his presidency he has become the embodiment of a unified national will. The ayatollah should understand this well.