The national debate over apartheid has changed -- permanently. We should expect that our government will, for a time, speak with a unified voice on how to prompt South Africa to dismantle apartheid. No longer will Congress and the administration be debating the means (constructive engagement vs. sanctions) of achieving this end. The question is now one of degree only.

When President Reagan decided to impose economic sanctions on South Africa, albeit very mild ones, his policy changed in principle. His overall objective has remained the same -- namely, to maintain a posture giving him the greatest leverage on the Botha government. Nevertheless, his imposition of sanctions was a de facto endorsement of the approach favored by Congress and the American people. There is a consensuAmerica on the means by which we should use our influence. This is a very significant development and one for which I commend the president.

The question lingers, however, of what our next step should be. Certainly we must give Pretoria sufficient time to respond positively, taking its domestic political situation into consideration.The United States has at its disposal very limited economic and political means to influence South Africa. By imposing sanctions, President Reagan has taken a decisive step toward implementing our economic leverage. We should now search for ways to use our political ties with Pretoria more effectively.

I would suggest that the president offer to act as mediator between a delegation of South Africa's white and nonwhite leaders. The parties would convene to grapple with means by which the nonwhite majority may begin to relize its legitimate aspirations to have a greater share in its own governance.

Unquestionably this would be a bold initiative. But it is not without precedent. The Camp David summit, which brought Egypt and Israel together, offers an encouraging paradigm. The Camp David accords proved that imaginative and creative leadership could accomplish the impossible.

President Reagan should already have the confidence of President Botha for such a summit. He should be able to establish credibility with black leaders because of his consistently harsh statements against apartheid and because of the enormous weight of congressional and public opinion that wants immediate liberalization of south African society and is willing to step up the pressure on the white government.

Botha would have to exercise enormous political courage to accede to a summit mediated by an outside party. But so did President Sadat and Prime Minister Begin. The political risks taken by Begin and Sadat are analagous, I believe, to those Botha would have to face. Botha has already reached the pinnacle of political success in his own country. He has much to gain, and much less to lose, should he venture forth to begin a process that should have begun decades ago -- real domestic reform.

Botha's challenge would be to convince his white countrymen, particularly on the political right, that the course of South African history has changed irrevocably. No longer can society continue as it has. In the past, social unrest was quelled with virtually no easing of apartheid. Today the black majority's patience has run out, and we may safely assume it will no longer tolerate domestic tranquillity at the expense of apartheid's permanency.

The time has come for both sides to agree to work together toward substantive social evolution to avert the prospect of violent revolution. The radical right of the white minority must concede that social evolution toward the dismantling of apartheid is inevitable. The radical left of the black majority must concede that violent revolution seeking the overthrow of the government would be disastrous, given the resolve and enormous power of the white minority and the toll such action would take on the black population.

If quiet diplomacy did generate intterest in a summit between Botha and key black leaders, the critical, perhaps unanswerable, question is whether these leaders can retain their constituencies. To what extent would Zulu Chief Gatsha Buthelezi continue to represent the nearly 6 million South African Zulus? Would they accept an agreement he accepts? Or what about Dr. N. H. Motlana, who, as chairman of the committee of Ten, presides over the elders of the explosive Soweto township? How much allegiance could Motlana command of the Committee of Ten and the Soweto blacks? For that matter, could the whites be relied upon not to replace the Botha government should they perceive Botha to be conceding too much?

These are pressing question, indeed, and the answers could well be negative. But we are faced with a bleak political stalemate that cries out for initiatives to break the deadlock. Responsible white and black leaders should be groping for answers, the whites not deluding themselves that they can continue in power just as they have in the past, nor the blacks deluding themselves that they can overthrow a powerful government without a blood bath and guarantee its replacement with a democracy that ensures basic human rights and political pluralism.

The fact is that with rare exceptions black Africa's indigenous people have not produced genuine representative democracies. This is a sad legacy. But it is one black South Africans (and all Americans) must acknowledge. And it is the context within which blacks must be prepared to negotiate with the white minority.

It would be a serious mistake, however, to place blame on the black majority for the crisis in South Africa. The whites are clearly culpable. They have had 40 years to at least draft a policy promulgating the eventual and peaceful transition to an integrated society. The policy they adopted and have subsequently enforced (apartheid) is the antithesis of what is reasonable, given the cultural and racial context of South Africa. Many whites apparently have never accepted the fact that transition is inevitable. If they do not change course and ensure that it comes eventually, they will ensure that it will not come peacefully.