The latest bad news, from Iran, offers this country an opportunity to make a fresh start on the hostage problem. The administration can break off the negotiations, assert a new freedom to take ominous action and let Tehran begin to worry about what Washington will do instead of the other way around. But such a change would compromise the con game that has served Jimmy Carter's domestic political interests so well.

The con game consists of the notion -- sedulously cultivated by the White House and the State Department flacks -- that Iran is run by a bunch of crazy weirdos capable of doing anything at any time. Matters are so delicate, the argument goes, that only the president can manage the affair. To criticize Carter on Iran is to seem to be playing fast and loose with the lives of innocent Americans.

The Iranian political situation in fact is not a mad scene. If nothing else, there is a clear top man. Ayatollah Khomeini, and he alone, has the power to loose the hostages.

The ayatollah, to be sure, has political problems. He rules as the well-nigh divine embodiment of a nationalist revolution. He has to keep the revolutionary fires burning bright to hold his followers behind him.

He is also the head of semi-modern state heavily dependent on trade with the outside world. He has to maintain some people in government who can manage affairs and keep other countries interested in doing business with Iran.

The hostages have at all times been the playing of that dural requirement. When they were seized back in November, the revolutionary flame was burning low in Iran. Troubles were breaking out on all the borders, and a moderate government under Prime Minister Mehdi Bazargan sought to assuage the difficulties by compromise. The seizure of the hostages, which the aytollah set in motion and then endorsed, put an end to that season of mellowness. There was a new surge of revolutionary fervor. The Bazargan government was outsted. The ayatollah swept a referendum for a new Islamic constitution.

Briefly the United States threatened economic sanctions and some kind of military reprisal. The Iranians then surfaced with Secretary General Kurt Waldheim the idea of a United Nations tribunal to assess their complaints against the shah and the United States. The Carter administration accepted the tribunal. Thereafter, working through President Abol Hassan Bani-Sadr, the ayatollah pushed the United States down a slippery slope where one retreat followed another.

First the United States implicitly accepted the principle of an international group sitting in judgment on the actions of past American presidents. Then it agreed that it would be appropriate that the tribunal be made up of Third Worlders and left-wingers sympathetic to the Iranian revolution. Then it accepted the idea that the commission would first look into Iranian complaints and then take up the affair of the hostages. Finally, it accepted the principle that the ultimate release of the hostages would be up to a parliament that has not even been elected.

Having won point after point after point, the ayatollah refused even to let the commission see the hostages. In desperation, the commission left Iran. Now time is out, and a new course is possible.

Breaking off the negotiations is the obvious course open to the United States. Washington could then sit back, arms folded, and say that since the Iranians had violated normal diplomatic rules in the most flagrant manner, there could be no further dealing until the hostages had been released. Simply by that step, the administration would at least arrest the steady erosion of the Amercian position. It would end the automatic, and costless, accretion of more and more power to the ayatollah.

Pressures, though better had they been applied earlier, could still be brought to bear. International transfer of funds can be made more difficult. Distribution of essentials inside the country could easily be sabotaged.

The mere threat of such measures would make the ayatollah and those around him feel the backs of their necks. Third countries -- notably the Europeans and Japan -- dependent of oil from Iran would carry the message that all kinds of trouble was in the offing unless the ayatollah became a little more reasonable.

Unfortunately, such a change cannot come easily to the administration. The president would have to admit in midcampaign that he has had no strategy for Iran except appeasement. He would have to acknowledge that he has been the principal dupe of the Iranian con game.