This week white South Africans vote on a set of constitutional proposals that Afrikaners see as momentous to their own identity as a race and to their whole future in southern Africa.
Not since 1948, when they first came to power and institutionalized apartheid (racial segregation), have Afrikaner Nationalists felt that so much was at stake.
The debate has split Afrikanerdom down the middle. On one side are those who, to quote Prime Minister Pieter W. Botha, accept that they must ''adapt or die.'' On the other, far right, side are those who feel that in giving a few inches, they forfeit the possibility of white survival.
To outsiders, however, the new proposals being put to the vote on Wednesday might appear as only modest reforms. They would bring aboard, for the first time in South African history, the minority Indians and Coloreds (people of mixed race) as junior partners in the previously all-white government.
Blacks, who constitute 80 percent of the population, are entirely left out. (Referendum spurs black activism: Report from South Africa, Page 16.) Instead, blacks remain relegated to their separate ''homelands'' - which constitute only a small fraction of South Africa's territory.
Some South Africans hope that offering some elements of power to Indians and Coloreds will prove a wedge opening toward similar steps later for blacks. But many blacks suspect that the time will never come - at least while whites hang onto the main reins of power.
For those blacks who dismiss the constitutional proposals as a fraud, the new political deal for Indians and Coloreds appears as a still more insidious form of apartheid. For in specifically excluding Africans, the government emphasizes its perception of blacks as inferior.
The effect has been to galvanize black political opinion, producing a degree of black solidarity not seen since the late 1950s.
While the debate impacts on both whites and blacks in South Africa, the results of the Nov. 2 vote have direct consequences for the Reagan administration's policy of ''constructive engagement'' in southern Africa.
Under the Reagan administration, southern Africa as a whole - with its strategic minerals and heavily used sea lanes around the Cape of Good Hope - has assumed far more geopolitical importance than under the Carter administration. Washington is convinced this region can be safeguarded against possible Soviet subversion only if all the countries in the region coexist for their mutual benefit and if their points of friction are removed.
An improved image for a reformed South Africa, economic benefits for hard-pressed African states, greater regional stability - these are the carrots the Reagan administration's policy dangles before the region.
High on the list of US priorities in the area is a Namibia (South-West Africa) settlement. Although it is elusive, Washington feels it is still in the cards.
Among the benefits for Washington from a Namibian settlement: removal of some 30,000 Cuban troops from Angola whose presence contributes to the Reagan administration's concerns about Soviet expansionism and the degree to which it is achieved through Cuban proxies.
It is Washington's belief that the only way the region can avoid what is its likely fate as a troubled ''Middle East of the 21st century'' is for these African states to work together without compromising their political principles. The administration feels such an accommodation is not feasible without an evenhanded US policy.
Washington believes it is more likely to induce change in South Africa by conciliation rather than confrontation. And change is seen as essential if constructive engagement is to have any meaning.
At the same time, the Reagan administration insists that it views South Africa's occupation of Namibia as illegal. It also says it deplores the homelands policy as an attempt to dump unwanted urban Africans into barren and inaccessible areas of the country.
For these reasons, Washington will undoubtedly welcome a positive vote in South Africa's Nov. 2 referendum, particularly if it nudges Afrikaners in a direction from which there may be no turning back.
A ''no'' vote would be seen as a setback to constructive engagement and a Namibia settlement, and as returning South Africa to a more rigid form of apartheid, possibly bringing the downfall of Prime Minister Botha.
The informed speculation around Washington is that the referendum will pass, although it may be close.
Chester A. Crocker, former director of African studies at Georgetown University's Center for Strategic and International Studies in Washington, is the father of the constructive engagement policy. He is tirelessly pursuing the doctrine in his capacity as assistant secretary of state for African affairs.
Countries falling within the policy's orbit are South Africa and Namibia, Angola, Botswana, Lesotho, Zambia, Zimbabwe, Swaziland, and Mozambique.
But the policy and Dr. Crocker are running into withering fire from a number of other African states whose leaders see the policy as a departure from the Carter administration's embrace of black Africa.
They also think US policy is providing diplomatic cover for South Africa to attack its neighbors and is failing to produce any substantial changes in apartheid. Many feel Pretoria is taking the US for a ride.
For their part, the South Africans deny the Reagan policy is pro-South African. They say it is more evenhanded, redressing an imbalance during the Carter administration.
A Namibian settlement would be the greatest prize of constructive engagement. It would remove the last chunk of colonial territory in Africa. Some Americans hope South Africa would start a process that could lead to eventual eradication of its pariah image.
Angola would, in turn, be freed of Cubans who - although deemed essential as deterrents to South African attack - have proved both unpopular and exorbitant to host. A settlement would open Angola to US diplomatic recognition, investment , and to assistance for its ravaged economy.
On the basis that constructive engagement provides something for everyone, the Reagan administration has greatly boosted US aid, economic assistance, and educational training programs to South Africa's neighbors.
In so doing, the US is sending a message to Pretoria that it has no intention of seeing its work in the region undone by any South African destabilization.
At the same time, Washington has alerted South Africa's neighbors (the ''front-line'' states) that if they harbor guerrillas or knowingly allow their countries to be launching pads for cross-border violence, they invite attack from South Africa.
The effect of this dialogue has been to drive a wedge between the front-line states and the liberation movements seeking to sabotage South Africa. The front-line states are becoming increasingly reluctant to take on South Africa. They have moved more in the direction of removing sources of provocation.
Those most antagonistic to constructive engagement appear to be: (1) the Kremlin, since the policy might lessen opportunities for mischief; and (2) rigid hard-line South Africans in the military and in the establishment.