S. Africa ruling-party split to test Afrikaner unity

''From unity strength.''

South Africans have given this message inscribed in their national coat of arms a new twist. Popular sentiment now seems to ask:

''Without unity what?''

The question arises in the wake of an important split in the ranks of the ruling National Party. This party is the political arm of the white Afrikaans community, which has governed South Africa since 1948.

Since coming to power, Afrikaners have seen political and cultural unity as a vital element in their survival - and in their control of a nation in which they are outnumbered by blacks.

What will ultimately result from this historic split remains unclear. But already it is plain that the division is more of a continuing fissure than a clean break. The splitting process is not over, analysts here agree, and it continues to test the underlying unity of the Afrikaner people.

The political break was brought on by Prime Minister P. W. Botha's acceptance of the concept of ''power sharing.'' He used the term to describe how whites might extend some political power to Indians and Coloreds (persons of mixed race) and actually share certain decisions with them over matters of joint interest.

Dr. Andries Treurnicht and 15 other Nationalist members of Parliament rejected in principle the notion that whites and ''nonwhites'' should ever govern jointly in any respect. On March 20 they formed their own Conservative Party of South Africa (CPSA).

Veteran observers here look on the split as the most dramatic evidence of a continuing tug of war that has been going on for some time in the National Party and the Afrikaner community.

It is between those favoring change and those favoring strict adherence to the apartheid doctrine of complete separation of the races. For most, the debate is over how best to ensure Afrikaner survival in the face of growing black nationalism.

''There is lots of confusion among the Afrikaners. People see the arithmetic of the growing number of blacks, and there is a sense that everything is not under control as it once was. The party split is just a manifestation of this,'' says historian Hermann Giliomee of the University of Stellenbosch.

One of the most significant changes in the Afrikaner community that could come from this split is a realignment of political loyalties. Should Mr. Botha now proceed purposefully with moderate reformist policies, including but going beyond the power-sharing plan, analysts reckon he will inevitably further divide the loyalties of the Afrikaner population.

But in so doing, he could conceivably gain support from the English-speaking community and broaden the National Party into a more conventional political party. With this, suggests Mr. Giliomee, might come a basic restructuring of white South African politics along economic and class lines instead of ethnic lines.

The other option for Mr. Botha would be to move very slowly or even backtrack on reform, so as to minimize the growth prospects of the right wing and Dr. Treurnicht's CPSA.

The risk with this policy is that it could alienate the reform-minded Nationalists who now support Mr. Botha. And since Mr. Botha is not apt to regain right-wing support, he could simply worsen the present problem faced by the National Party of dwindling support on both the right and left, evidenced in the last general election and in recent municipal elections.

Dr. Treurnicht has dedicated the CPSA to strict political segregation. The party's strength is difficult to gauge. It drew a crowd of 7,500 at its opening rally.

But it begins with some clear handicaps. It does not have the support of any of the important Afrikaans newspapers, which has deprived it of a platform for its views.

Also, CPSA is not seen as having the potential to ignite an ''ethnic revitalization'' campaign, as the Afrikaner community is not homogenous enough today to rally around a theme of renewed ethnic identity, most analysts here believe.

Still, it is clear Mr. Botha and the National Party take the split seriously.

Recognizing the turmoil within its own ranks, the National Party has taken the unusual step of calling party members to a national congress later this year in advance of the usual provincial congress. Power sharing and other issues will be discussed.