Step right up, folks. Here we have two shells and under them we'll put two peas. We'll call one of them South Africa and the other Russia, and we'll announce two crystal-clear policies for each: Constructive Engagement for South Africa and what we'll call Unmitigated Hostility for Russia. Now keep your eyes on the shells. Music maestro, please.

We'll move 'em around. We'll communicate in a way that could only be called great and keep up a line of patter that distracts most everyone from noticing what's happened. Now we pick up the shells and under the Soviet Union shell is the Constructive Engagement pea and under the South Africa one is Unmitigated Hostility. Ta, da!

Actually, this particular shell game has been going on for some time, but the lifting of the shells took place only recently -- International Human Rights Day. In a speech marking the occasion, President Reagan sounded downright moderate about the Soviet Union. Abandoning the harsh tone he has used in the past ("evil empire,"etc.), Reagan merely took a routine jab at Russia. He said it had a human rights problem.

But what kind of problem? Here the president was at a loss for words. He did not mention a single dissident, not even Andrei Sakharov, nor the plight of Soviet Jews -- up to now absolute boilerplate in any speech dealing with Soviet abuse of human rights. No more of that. Now we would prefer to talk about these matters in private. As Barry Goldwater might say, none dare call it d,etente.

Whatever you call the president's new tone toward Russia, the one he has adopted toward South Africa cannot be called Constructive Engagement. "The inhuman policy of apartheid continues," Reagan said. He went on to deplore the "essentially unlimited powers" granted the police and condemned the jailing of South Africans (most of them nonwhite) without "even elementary judicial protection." This from a man who only last summer stuffed a mighty boot in his mouth by hailing South Africa's progress at eliminating racial segregation.

It is probably too much to say that South Africa and the Soviet Union have switched places in Reagan's foreign policy cosmology. It is not too much to say, though, that the policies toward both countries, once based on certainty, have not only changed considerably but are in disarray. The reason is not hard to discern: in neither case did the policies work. For instance, neither tough language nor moral reproach has persuaded the Kremlin to allow Soviet Jews to emigrate, grant Sakharov the privilege of living where he chooses or enticed the Soviets to withdraw from Afghanistan.

Similarly, constructive engagment (d,etente by another name) resulted in no fundamental changes in apartheid. South African blacks still cannot vote, have no representation in parliament, cannot live where they choose, have to carry internal passports, are educated in segregated and inferior schools and live at the mercy of the police. Constructive engagement's only achievement -- and it is an unparalleled one -- has been to convince many Africans that the United States is either sympathetic to or supportive of a racist, repressive regime.

The mutations of these policies are not just twitches in Washington's ongoing effort to make some sense of the world. They are instead unacknowledged and maybe inadvertent reappraisals of the fundamental convictions that are central to the Reagan foreign policy. Both policies reflected deeply held conservative beliefs (Russia is implacably evil; South Africa is an anti- communist ally) and a simultaneous contempt for the way Jimmy Carter (his grain embargo notwithstanding) had handled the same problems. Carter, it was said, had done it all backward -- hectored Pretoria and made nice-nice to Moscow. They must be serving peanuts nowadays in the White House mess.

As usual when policy evolves, there are two steps forward and one back and the standard denial that anything has changed -- least of all deeply held views. But if you look under the shell labeled Soviet Union, you will find Constructive Engagement and if you look under the one labeled South Africa you will find Unmitigated Hostility. It's the way the shell game is played in Washington. Once again, the mouth is faster than the eye.