THE FOCUS OF THE bargaining over the American hostages in Tehran has shifted, at least temporarily, from the international stage to the internal scene in Iran. The United States has not stopped trying to isolate the kidnappers in international opinion -- an effort in which it has achieved gratifying success -- and it has not eased the direct pressures, economic and other, it is applying on them. The Iranians holding the prisoners are still intent on conveying that they will hold firm until they get the shah in their hands. Yet it is among Iranians that the principal negotiating now seems to be going on: the politics of the bazaar.

The crisis has produced fissures between the students actually holding the hostages, the officials appointed by Ayatollah Khomeini to replace the government he forced to resign, and the ayatollah himself. They appear to differ on the terms to offer the United States and the tactics to use as well. Their differences are coming increasingly into public view.

This development is not so surprising, given the structure, or lack of structure, of the Iranian revolution and the extremity of the circumstances that now prevail. Its practical effect is to set aside all other political considerations in Iran, for the duration, and to make this confrontation with the United States the field -- a highly visible field -- on which to play the revolution out.

The students, the leaders on the left, seem ready to pay any price to win and may even believe that victory means a measure of revolutionary purity that can be attained only if the crisis is pushed to the limit. The new ministers, in relative terms the moderates, appear to be calculating the price; they convey the impression they would like the crisis to end. The ayatollah's announcement that he is tired and ill and will receive no callers for three weeks leaves outsiders to ponder whether he is faking it or not.

That there is now no evident authority in Tehran, no single source in control and available for bargaining, would seem to complicate the administration's task. But the cracking of Iranian solidarity may yet open up new negotiating possibilities. The hard-line faction in Tehran not only is almost completely isolated in terms of the world; it is also somewhat isolated just in terms of Iran. As the costs to Iran increase, it may become more so. This holds a potential that the administration cannot avoid trying to understand and exploit.