THE QUESTION KEEPS coming back. How can there be an oil stortage when there's plenty of gasoline for sale at the filling stations? The lights - and much more important in this season, the air conditioners - go on at the flick of a switch. Statisticians report that the world's present capacity to produce oil is even greater than today's soaring demand.Yet the White House is muttering about a standby plan for gasoline rationing. Where's the crisis?

The answer lies in this country's heavy and rapidly growing dependence upon Saudi Arabian oil production. The Saudis are the only source in the world with the capacity to keep raising production in pace with the imports that the industrial countries expect to need over the next decade. There is now only one country with the power to decide whether there is to be an oil surplus or an oil shortage in the late 1970s and early 1980s.

That truth is, you might say, the dark side of President Carter's energy plan. It's the part of the plan that he can't very well talk about openly. The United States is currently trying to persuade the Saudis to do three things that are politically very difficult for them. First, the United States wants them to continue holding back the rate at which OPEC raises oil prices. Second, the United States wants them to increase production to meet the world's demands for oil. Third, the United States wants them to raise their production even beyond current demand in order to fill the American strategic reserves.

In return, the Saudis need military and political protection, for they live in a dangerous part of the world. But Mr. Carter is not in an altogether solid position to make promises. It's not clear what, if anything, the United States could do for the Saudis if , for example, fighting broke out in the Persian Gulf region. Even if the Saudis were to decide only to maintain their present production level, continuing to pump but refusing to expand, shortages would rapidly begin to develop throughout the world.

There aren't any easy outs. Oil from Alaska will certainly help. But it will replace only about one year's increase in imports, if American consumption keeps rising at the present rate. Similarly, the North Sea oil will reduce European imports for several years but then, by most forecasts, the numbers will start marching upward again. Japan can hardly avoil continuous increases in oil imports.

The faster the world's demand rises, the more visible and vulnerable the Saudis' production policies will be. Other OPEC countries see high Saudi production as a threat to prices. Other Arabs know that the American strategic reserve is designed to blunt the oil weapon. Even within the Saudi government there are evidently voices urging greater conservation of the country's single resource.

The entire American economy is now balanced precariously on the Saudis' willingness to keep production high and to keep raising it. This enormous power has been suddenly thrust on a small country, run by a feudal monarchy, with no defense capability, in one of the world's most unstable regions. It is a country already earning more dollars than it wants or can spend. Does that constitute a crisis? It certainly helps explain why the administration is talking about rationing at a time when the country is awash in gasoline. What are the chances that the Saudis will change direction? Nobody knows. But with every additional pint of oil that the United States requires from them, the risks rise.