PRESIDENT Frederik de Klerk of South Africa wants to mold a constitutional system that combines majority rule with minority rights. It's a pattern that has worked in other parts of the world, including the United States. But the balance struck between various interests has to reflect the realities of the society that will have to live with the new structure. And the dominant feature of South African society is apartheid's legacy of distrust and disenfranchisement.Blacks in South Africa, who make up over 70 percent of the population, still are without the vote. When they finally exercise the right to cast a ballot, they don't want it hedged about by built-in white vetoes. The constitutional revisions put forward by Mr. De Klerk this week hint, at least, at such vetoes. A upper chamber of parliament chosen through regional representation could be manipulated to give the country's white minority disproportionate power. Likewise a presidency that would consist of a committee of political leaders rather than a single executive. Local councils elected partially by property-owners would obviously tilt toward the relative few who have the means to own land. The African National Congress, predictably, rejected the De Klerk plan out of hand. But the president's announcement was only an opening move in a political contest that should now move toward formal constitutional negotiations among all South Africa's varied constituencies. De Klerk has an interest, after some recent political setbacks, in showing his moderate white backers that he still shares their concerns. He knows that the structure he's proposed will go through alterations as talks get under way. The same is true of the ANC's negotiating positions. A final product will have to give the majority of South Africans unequivocal ability to elect leaders who can set policy. It will also have to include broad safeguards of individual rights - which could also be the best way to protect the legitimate interests of groups.