SO PRESIDENT CARTER has decided to declare a holy war against Big Oil, as a device to get the windfall profits tax through Congress. Somehow it isn't convincing. The president charges the oil companies with plotting large and costly loopholes in the windfall tax. But he would be in a stronger position in attacking Big Oil if the administration's own intentions were clearer. Although he called for the tax nearly three weeks ago, in his television address, his administration has yet to produce a precise description of what it wants. The Treasury Department is supposed to supply some of the details and definitions to the House Ways and Means Committee today. But it will be some time, apparently, before an actual bill is in hand.

Mr. Carter is getting tangled up in his own indignation and his own logic. He called for decontrol of oil prices to provide incentive to the industry to find and produce more oil in the United States. The oil industry, never one to let well enough alone, suggests that there might be a plow back-a tax exemption for profits reinvested in exploration and development. In response, Mr. Carter angrily accuses the oil companies of trying to "hookwink the American people," and declares that the plow back is merely a "kickback." Listening to him, people might well ask: But wasn't the whole idea to create new incentives?

It's a matter of degree. Under controls, some American oil is still sold for less than $6 a barrel. Through decontrol, and a steady rise in world prices, oil from the same wells may be going for $20 a barrel by late 1981. Mr. Carter is proposing, essentially, that the companies keep half of this tremendous gain as an incentive-and the windfall tax will take the other half.

That's sufficient incentive. In Britain's North Sea, taxes take about three-quarters of the difference between revenues and production costs. In Canada, taxes take about two-thirds. In both places, drilling goes forward merrily-at the hand of the same companies operating here. It's worth nothing that revenues to both the oil producers and to the federal government are going to be much greater than the administration's estimates, because they are based on unrealistically low estimates of future world oil prices.

Instead of making a reasoned case for a reasonable policy, Mr. Carter is including in bad-tempered accusation and terms like "kickback". That kind of rhetoric has a great capacity to sour the process of negotiation and compromise that lies ahead of any tax bill. The president will certainly get his windfall tax, but the holy war, and the passions it arouses, can only make it more costly.