The recent spurt of support for Jimmy Carter asserts a fundamental fact of American life: ours is a presidential nation.

In times of trouble and confusion, the country does not look to Congress or the courts or to some ruling establishment. It turns to the White House for a single, responsible leader.

The Iranian crisis presents a case particularly to the point. For it involves lining up clashing objectives in a context of conflicting local, regional and global interests.

The immediate object is to spring the hostages. But the hostages are being held by a kind of revolutionary commune that is not altogether responsive to a government that is, in itself, chaotic. The only figure with enough authority to order the release is Ayatollah Khomeini. So for the time being, the United States has an interest in his continued role.

But this country also has an interest in fostering a more responsible government in Tehran. The present regime not only patronizes disruptive incidents such as the seizure of the embassy. It not only breaks up the established pattern of trade in a vital commodity.

It also distills and spreads a brand of Islamic fundamentalism that menaces stability in the neighborhood. The poison threatens directly the highly vulnerable regimes in Saudi Arabia and the other sheikdoms of the Persian Gulf that play a crucial part in the international economic system. It could menace the regime of Anwar Sadat in Egypt -- the cornerstone of the American presence in the Mideast.

Given that malign influence, the Khomeini regime cannot simply be isolated. Nor can the case of the hostages be allowed to drag on month after month as did the case of the crew of the Pueblo, seized by North Korea. On the contrary, the United States has an interest in the early replacement of the Khomeini regime.

Even if Khomeini is not pushed, moreover, he is bound to fall soon. He is both old and ailing. So the question of succession has to be faced. For not anybody is just dandy.

Iran occupies a strategic piece of real estate. The borderlands around the central plateau are populated by ethnic minorities never easy under rule from Tehran, and now visibly in the process of breaking away. Their unrest harbors the potential for spreading trouble to Iraq, Afghanistan, Turkey and Pakistan. The whole tier of states on the southern fringes of Russia could go agog with internal strife.

So the United States does not want the ayatollah to be replaced by any of the Marxist figures in his entourage. Neither does it want what is much more likely -- the emergence of a military firebrand mixing Islamic fundamentalism with narrow chauvinism in the fashion of Col. Muammar Qaddafi of Libya.

On the contrary, the United States needs a moderate regime in Tehran. The American interest favors a government based on senior military people and including such figures as the temperate religious leader Ayatollah Shariatmadari.

Lastly there are the American connections with the allies and the Soviet Union. The stake of the Germans and the Japanese in a benign outcome in Iran exceeds that of this country. The French have political assets in the area, and a have-gun-will-travel outlook on international politics. While the allies may cast envious eyes at this country's preferred position with access to the oil of the Persian Gulf, high-level consultation in joint political effort remains essential.

As to the Russians, the world's leading conformists can hardly be happy with the seizure of the embassy. In the past Moscow and Washington have joined in cooperative measures regarding Iran. It seems highly unlikely that the Russians -- in the presence of a chance to wipe out the whole American position in the Mideast -- would now be prepared to behave in a responsible fashion. But the possibility must be explored.

What all this means is that the Iranian hand is exceedingly hard to play. Only the president can do it. So for my own part, as a not overly gentle critic, I am heartened by the rallying of support for Carter.