IT IS GOOD to have President P. W. Botha joining the current discussion about change in South Africa. He is a knowledgeable and necessary interlocutor, and he has two important roles. Not only does he speak for his country's enfranchised white minority, but he also rules, through the forcibly imposed apartheid system, the voteless black majority. Precisely in the tension between those roles his leadership now faces its hardest test.

Politically, Mr. Botha has a dilemma. To satisfy his white constituency, he must uphold white interests as they are variously perceived; this includes maintaining the image of total Afrikaner or at least white control of white destiny, although whites long ago lost that control. Hence his pouting and unpersuasive rejection of the suggestion by President Reagan that it was American "quiet diplomacy" that led Pretoria to release some detainees.

At the same time, to keep the connection with Washington that spares Pretoria unbearable loneliness in the world, President Botha must show a certain progress in dealings with South Africa's blacks. From his point of view, the effect of the demonstrations begun last month in Washington can only have been to raise his domestic costs of propitiating Ronald Reagan, since, notwithstanding his own protestations, Mr. Reagan is being forced by the demonstrations to demand more of Mr. Botha than he has in the past four years. Just how much more, and in what forms, will be determined in the months to come.

Meanwhile, we can expect pronouncements from South Africa -- that is, from the white government, an important but not the sole voice and actor -- along three lines:

* South Africa is strategically and economically vital, or at least awfully useful, to the United States. This is certainly true, but the formulation begs the question of whether it is wise for Americans to count on a regime that may be increasingly distracted by internal unrest.

* South Africa's internal arrangements are not our business. But if we are invited not to care for the blacks and the Asians and the Coloreds, why should we be expected to care for the whites?

* South Africa's internal arrangements are our business, but we should understand it's working earnestly to change things for the better. In fact, it's working hesitantly to change things to uncertain purpose. Changes made or proposed do not cut squarely, as they must, across the dehumanizing and denationalizing of blacks that are the essence of apartheid.

"We pledge here today," Mr. Reagan said last Monday, "that if South Africans address the imperatives of constructive change, they will have the unswerving support of our government and people in this effort." But only if. Otherwise, all bets are off. The pledge reflects, we believe, an American consensus. It deserves the closest reading in Pretoria.