The South African government is bargaining with Nelson Mandela, the black South African who has been a political prisoner for 21 years, over the terms of his release. It is a development with historic potential: if the government frees Mr. Mandela, the way will be open for blacks to join whites in an unprecedented search for a political society acceptable to them both. It could mean the beginning of the end of apartheid.

Mr. Mandela is no ordinary prisoner. He is the leader of the outlawed African National Congress, the main black underground group and a likely candidate to become, again, if it is legalized, a major national political organization. He has been in prison longer than most South African blacks have been alive, and he is a legend in his own time. This is due to the principled constancy of his anti-apartheid stand, and to the courage of his wife and, now, his daughter in pro and they have never stopped insisting that, despite the immense personal hardship, he will not accept release unless he is permitted to take up a full political role. His stature is at once what allows him to drive a hard bargain with the government now, and what makes it worth the government's while to engage him as an interlocutor.

President P. W. Botha at first offered to let Mr. Mandela go, as a private citizen, to a black "homeland." He refused. Then Mr. Botha offered release in return for a pledge to "unconditionally reject violence as a political instrument." At the same time, Mr. Botha allowed Mr. Mandela two unusual forums: last month, an interview in a London newspaper and, last Sunday, a public outdoor rally in Soweto. There Zinzi Mandela, his 22-year-old daughter, defied a law forbidding the public quoting of her father and read a message he had dictated in prison and had been allowed to pass on to his wife, Winnie, herself a "banned" person. Its essence was that he will accept release only to represent his people in an open and dignified way, and that he will make no promises until they are free. Thousands cheered.

It is up to Mr. Botha, an Afrikaner carrying the burden of his people's lonely and insupportable racial inheritance. As he hesitantly opens the door to minimal reform, others seek to pry it open wider. By moving toward a dialogue with blacks he risks shredding his white constituency. He and his fellow Afrikaners, to escape an ever harshening racial confrontation, must reach deep within themselves and take a breathtaking chance on racial partnership. No one can assure them that taking the chance will produce a good result. They can only be assured that for them there is no alternative.