President Carter has enjoyed a fabulous response to his call for support against the Soviet invasion of Afghanistan. Practically the whole of this country backs him, as does most of the world.

But Carter has yet to commit American power on the spot. Without such a commitment, without engaging the deterrent, all the actions so far taken are a disservice to this country, its friends and even the Soviet Union.

To understand this paradox requires another thing the president has not vouchsafed -- a diagnosis of what went wrong in the area around the Persian Gulf. Basically the trouble goes back a year ago to the fall of the shah of Iran. His departure left a vacuum at the vortex of world power.

In Iran itself there took office an Islamic regime that lacked the capacity to govern. Not only did things fall apart in Tehran, but revolts flared among the ethnic minorities in all corners of what used to be an imperial realm. There was trouble across the tier of states running east from Turkey and Iraq to Afghanistan and Pakistan. Equally down the Persian Gulf from Kuwait to Saudi Arabia to Oman, Yemen and the Horn of Africa.

Constraining that kind of trouble is a distinctive obligation of the superpowers. But the Carter administration ducked the responsibility. It had no sense that the fall of the shah entailed the gravest security problems. It pretended that nothing untoward had happened when Ayatollah Khomeini came to power. Even now -- when the affair of the hostages reveals the inability of that regime to govern without inflaming revolutionary passion -- the Carter administration imagines it can do business with Tehran.

The Russian, when Islamic fundamentalism disturbed their interest in Afghanistan, reacted in a much different way. They sent in troops, staged a coup and occupied the country. Those moves were certainly brutal. But for the time being, the Russians control Afghanistan and are in position to exploit whatever troubles may develop in the neighborhood.

Carter's response has been one of outrage. He has called for a wide range of actions that condemn the Russians and raise the threat of punishment.

Of this country, he has asked the suspension of trade with Russia, including the embargo on grain sales. Boycotting the Olympics has been made a kind of moral obligation. Much higher defense spending and a step toward the draft have been put before the Congress. Virtually everybody has given their accord.

Japan and the European allies, despite a heavier political and economic stake in good relations with Moscow, have joined the United States in condemning the Soviet Union. The Moslem countries, at the Islamic conference in Pakistan, sunk differences on Israel and differences with respect to the role of religion and united in opposition to the Russians. A broad range of large countries in the Third World have also thrown in their lot with the United States. China has agreed to supply the resistance in Afghanistan with anti-aircraft weapons and machine guns. Even India, though rightly concerned about a Pakistan supported by the United States and China, has questioned the Soviet moves.

But the quintessential ingredient is missing. The Carter administration has not yet faced up to its responsibility as a superpower. It has not accepted an obligation to maintain order in an area now critical to international stability. Neither has it put American ships or planes or soldiers on the spot in the threatened area.

But unless steps of that kind ar taken, everything else comes apart. The American people are bound to ask whether they aren't being led into a meaningless war dance. This country's allies and friends will of course recoil from sacrifices the United States refuses to make. Even the Russians will have reason to believe that Washington is neither insuring against the kind of unrest that is bound to draw them toward, nor invoking against them the one thing that truly does deter -- namely, the deterrent.

Perhaps the requisite measures are being taken now in the current visit of presidential envoys to Pakistan, India and the Persian Gulf. The obvious quick step is to put an American naval air squadron on the spot right away. But everybody has reason for doubt. So far at least, the president, the secretary of state and the secretary of defense have not yet shown they have conquered their deep-seated aversion to the idea that the United States must behave as a superpower.