One advantage of a campaign year is that you don't really have to think about why a president running for reelection is doing what he does. Consider the Iranian hostage issue.It's simple.

At the start, when public outrage was brought to a white heat by the cameras at the Tehran embassy gates, the president made the hostages the center of his foreign policy concerns. His approval ratings soared. But when the rescue effort failed and the hostages began to be a domestic political embarrassment, he played it down, because his approval ratings had plummeted.

This is supposed to explain why Carter made almost nothing of the hostage issue at the Venice summit, and why Secretary of State Muskie did the same at the NATO defense ministers meetings later on in Ankara.

Except that it doesn't. A much more obvious reason can be found here (and in other allied capitals as well). The British think much too much was made of the seizure of the hostages from the beginning. They don't like the economic sanctions. They shudder at the thought of some new use of military force.

For Carter to have pushed the hostage question in Venice would have been to set himself up for a fall at home -- and in Tehran. A damaging display of allied disunity would not have been lost on the Iranians.

Aha, you say, but these are not British hostages and the British have their own domestic reasons for wanting nothing to upset business-as-usual with Iran. True enough. But the troble with applying the domestic political theory of the case of everything is that it tends to obscure to discredit what may be, on their merits, entirely sensible arguments in strictly foreign policy terms.

The arguments I'm talking about -- for a policy of patience as the surest, shortest route for securing the hostages' eventual release -- come from diplomats, civil servants and oil company specialists in Iranian affairs.They happen to be shared by a good many American experts. But these arguments have nothing to do with domestic politics in either Britain or America -- and everything to do with internal politics in Iran.

From the first day of the hostages' captivity, there has been nothing that could properly be called a government in Tehran. This became all too painfully evident at the end of last March, with the collapse of the Carter administration's torturous, circuitous diplomatic effort to arrange a transfer of the hostages from the so-called militants at the embassy to government control. It is no less the case today.

"There is simply nobody at the other end that you can count on," says one American authority, "nobody who can take responsibility. That's been the central problem from the beginning."

There is, of course, Ayatollah Khomeini, an aging, mean-minded religious fanatic who rules by mob and mystique. But the impression that he has the last word is not the same thing as saying that he can rule.

There is now an elected Majlas (parliament), but it has never met except in rump sessions, without a quorum, and it is incapable of anything but arguing over how to organize itself. There is President Bani-Sadr, a relatively reasonable-sounding fellow, whose resignation is in Khomeini's hands and who openly accepts no responsibility for the actions of his own ministers.

What useful message could the Venice summiteers or the allied ministers in Ankara have sent to such as Iranian "government"?

One American specialist estimates that the achievement of an effective central government in Iran "may take years." The British talk even more grimly about the implications of Iran's disintegration into ancient regional entities.

By this reckoning, the best hope for the hostages lies in discreet dealings with some relatively moderate factions, sufficiently alarmed by the undeniable impact of the sanctions on the Iranian economy and persuaded that the hostages are a liability, if not a genuine obstacle to political stability.

This is a painful conclusion to reach. It's an even more painful policy to preach. It is not easy to explain to Americans in an election year that: in resolving the hostage crisis it is not so much the United States as Iran that is powerless.