IN LINE WITH the Carter administration's division of diplomatic labor on Africa policy, Vice President Mondale has been working the white side of the street and United Nations Ambassador Andrew Young the black side. Their recent activities illuminate the dimensions, and even more the difficulties, of what Mr. Carter is trying to do.

Mr. Mondale, after meeting with Prime Minister Vorster, reported tentative success in gaining greater South African cooperation in Rhodesian and Namibia. But he admitted encountering "fundamental and profound disagreement" with his suggestion that, under pain of losing official American favor, Pretoria start abandoning apartheid. He and his chief, and Mr. Young, seem to think that apartheid is expendable in South Africa, as segregation was in the American South, and that, if the white minority reliquished it, its position would still be tolerable, perhaps even in some ways improved. But most South African whites appear to believe that, though it may be possible to remove the rough edges of discrimination in public facilities, separate development is the only basis on which a white minority can be sustained at all.

Mr. Young, at a U.N. decolonization conference, elaborated on the administration's assumption that the nonviolent tactics used in the American civil-rights struggle can overcome apartheid. But his black African listeners were skeptical. They see vast differences between the United States and South Africa. In the former, a black minority operated with powerful white allies and with the spirit of the country, the intent of the law and the power of the government essentially on their side. In the latter, a black majority operates with few white allies and with the prevailing national spirit, the law and the government essentially opposed.

President Carter is proceeding, in our opinion, on a mistaken view of the nature of the problem in South Africa and of the role open to the United States there. Pretoria's readiness to help out on Rhodesia and Namibia in order to buy tolerance or time for apartheid - notwithstanding Washington's stated refusal to make that deal - obscures but does not eliminate the Carter misperception. It would be gratifying indeed to believe that American pressure on Pretoria will improve the situation of South Africa's blacks. But it is at least as likely that American pressure will simply intensify the building racial struggle without contributing constructively to its settlement. In openly attacking apartheid, American policy is being placed at the service of American values. But where is it taking the people it is supposed to benefit, and where is it taking the United States? Mr. Carter has yet to say.