IF CONVENTIONAL political logic held in South Africa, the long-ruling National Party would have been severely punished at the polls for having brought the country into its current state of domestic and international crisis. But instead Prime Minister John Vorster played masterfully, and cynically, on his white constituents' fears of racial change and their anger at international pressure and scored a landslide victory. He can now fairly claim an overwhelming white mandate to continue a policy of repression and slege. Many of his adversaries expect him to.

But is this Mr. Vorster's plan? That he has a mandate is this plain. But that he is inclined or compelled to construe it as a mandate to indulge the policies that have brought South Africa toward disaster is something else. In fact, Mr. Vorster has a great opportunity. He has not only shredded his liberal opposition and with it the threat - always alarming to the Afrikaners who dominate South Africa - of seeing the "enlightened" among them split off and go with the mostly English-speaking opposition. He has demolished the Afrikaner far right, as represented by the Herstigte Nasionale Party, and with it the equally alarming threat of seeing the "conservative" wing of the Nats split off and go right. In brief, Mr. Vorster has honored the cardinal Afrikaner law: He has kept the Afrikaner political community united. And having done that, he can take the Afrikaners just about anywhere he wants to go.

What is John Vorster's choice? During the election period, the Afrikaners subordinated their own intense ongoing debate on the future of the country to the requirements of Afrikaner unity. That debate is the best thing going in th white politics of South Africa in 1977, and it is bound to become vigorous and pointed now. It is Mr. Vorster's style to stay close to his political base, to make what changes he does make in a manner calculated to disarm those to his left and calm those to his right, to scuttle sidewise like a crab. Yet he has persuaded some of the South Africans, white and nonwhite, who are most sensitive to their country's needs that he has the sense of history and the perception of Afrikaner self-interest needed to start moving clearly and irreversibly toward a more just society. John Vorster is, after all, the man who believes that a failure to so move would produce results "too ghastly to contemplate." Let him show his people, of all races, he means it.