IT'S NOT SURPRISING that the Carter administration is having trouble getting its crude-oil tax through the Senate. This huge tax is devised to remedy two dangerous defects that most Americans have never heard of. First, it would end the large subsidies for imported oil. As repeated polls have shown, hardly half of the American public ia aware that the country even is importing oil. How many people do you suppose understand that high-cost imported oil is being subsidized, as it passes through the refineries, by the producers of cheap, price-controlled American oil?

That leads to the second of these dangerous defects. The subsidy operates through a gigantic, unweildy system of accounting units known as entitlements. Some $200 million in entitlements changes hands every month. The system is extremely difficult to police and would be easy to mainpulate. If there were no other reason to get rid of the creaking entitlement machinery, the possibilities of fraud and scandal would be enough.

The entitlements are an attempt by the government to hold oil-industry competition on an even keel. The price controls keep most domestic oil selling for barely one-third as much as imported oil, and some refiners have much more access to domestic oil than others. To equalize costs, the government requires refiners with the cheap oil to subsidize the others by buying entitlements from them. This subsidy never goes through the U.S. Treasury, and it appears in no public budget. But it's more important than many that do. A subsidy of $200 million a month for imported oil helps explain why Americans keep using more of it than the country can afford.

The crude-oil tax, a basic element of President Carter's energy plan, offers a relatively simple solution. Producers would continue to sell at controlled prices, which range as low as $5.25 a barrel. But over the next several years the tax would raise the cost of all oil to the world level, now about $14.50 a barrel. The effect on consumers is not altogether easy to forecast, but the best guess is that they would face an increase of around 7 cents a gallon. On home heating oil, the tax would be entirely rebated. By 1981, it would raise at least $11.5 billion a year.

The attack on this tax is coming from two directions. Some of the old-line consumers' groups are still in the grip of the fixed idea that the lowest possible price always represents the highest public interest. Most of the oil companies are also fighting the tax - for precisely the opposite reason. They have a rather clearer view of the situation, and they have a rather clearer view of the situation, and they understand that this crude-oil tax, or something like it, is essential to the perpetuation of price controls. They are against controls, and consequently they are also against the tax.

The administration has greatly contributed to its own present difficulties by being very coy and secretive about its plans for this enormous flow of revenue. Rep. Al Ullman, chairman of the House Ways and Means Committee, has said that the money would be used to get the President's forthcoming tax-reform bill through Congress. That's not a bad idea. But because the administration remains silent, the Senate Finance Committee is now busily proceeding to work up its own plans - most of them pretty dubious - for those billions of dollars.

The crude-oil equalization tax is basically a sound idea that deserves to be enacted. It would increase prices to consumers by only a modest amount, and only gradually. This tax represents a necessary beginning to the hard job of ballasting the American economy against oil shortages, natural or contrived, in years to come. The tax's opponents have no better policy to offer - just a chorus of cries for lower prices to consumers, on one hand, and higher prices to producers on the other. You may have noticed that those two claims do not fit together very conveniently.