ROBERT MUGABE does not formally become prime minister until April, but his imminent ascent to power in Zimbabwe has already transformed the political landscape in neighboring South Africa.

Among the non-white South African majority, there seems to be elation at Mr. Mugabe's triumph, and an appreciation of the key role that armed struggle played in positioning him both to gather popular favor and to enter the elections he finally won. At the same time, many blacks understand that easy comparisons of white Rhodesia, which was finally forced to yield power, with white South Africa, which has the will and means to make black armed struggle incredibly costly, are unwarranted. This is quite apart from any hopes that the new Zimbabwe will sponsor armed resistance in South Africa; such hopes would appear to be doomed by Zimbabwe's desperate practical need for tranquillity and for good relations with its neighbor to the south. Among non-whites in South Africa, debate may now intensify between those who believe armed struggle to be the only or the best way to liberation, and those inclined to test new prospects of negotiating political change.

Among the white minority, the response to Mr. Mugabe is dominated by acute awareness that what happened and what may happen in Rhodesia/Zimbabwe is probably the closest thing there is to a model of change for South Africa. For them, the basic unpalatable fact is that the white minority lost power. Relief that this transition was peaceable in its final stage is tempered by recognition that it took a brutal war to produce the elections -- and that Zimbabwe's whites face an extremely uncertain future. South Africa's prime minister, P. W. Botha, has courageously seized on this moment of white consternation to propose a "state conference" in which for the first time all racial groups, including urban blacks, would be consulted on the country's future. His initiative is forcing a fateful debate on whether whites should move beyond compromises meant merely to buy time and divide non-whites on racial, tribal or economic grounds, or whether they should move deliberately to grant equal rights to all South Africans.

Americans of various political persuasions, looking now at South Africa, may feel themselves tugged to offer counsel or otherwise try to influence the outcome. It is an impulse to be resisted. The upheaval in South Africa's own back yard has made the parties sensitive to the requirements of their situation. They do not need to be told long-distance, by people who will not have to share the consequences, to embrace either revolution or reform. They seem, finally, to be on the way to making their own choice. For the effort, they deserve our respect.