S. African local election will test plan to share power with Coloreds, Indians

A political drama unfolding in South Africa could become a test of public opinion on the government's plan to include Indians and people of mixed race in Parliament.

The opening act was a heated argument across the floor of Parliament between Minister of Manpower S. P. ''Fanie'' Botha, noted for his progressive labor policies, and Andries Treurnicht, South Africa's chief ideologue on the right, and the government's most worrisome opponent.

The argument turned into chest-beating challenges for each to resign their seats in Parliament and stand for reelection in their separate Transvaal province constituencies. Although the challenges were apparently spawned in the heat of the moment, neither would back down even after tempers cooled.

The result: South Africa is bracing for by-elections that no political party particularly wanted, but that could nonetheless be highly significant to white politics in South Africa. The by-elections are expected some time in May.

More than just the careers of two individuals are at stake. The by-elections have been elevated in public perception to a referendum on the government's limited ''reform'' policies, principally the plan before Parliament to bring Coloreds (persons of mixed-race descent) and Indians into the central government.

It was this limited ''power-sharing'' proposal that led Dr. Treurnicht to leave his Cabinet post last year and form the new Conservative Party in opposition to the government.

The expectations - and worries - aroused by the Conservative Party have largely subsided. The party has been so far successfully isolated by the Nationalist Party government and the Afrikaans press, and has yet to win any political victories that would signal voter groundswell.

Also hampering the Conservative Party is its inability to join forces with the smaller, further-right Reconstituted National Party. Combined, the two would probably pose a formidable right-wing front, but apart they divide the ultraconservative vote, leaving the middle ground to the National Party.

It is the already weakened state of the Conservative Party that makes many National Party strategists wonder whether the by-elections are worth the inherent risks. A strong Treurnicht victory could restore momentum to the young party. A Botha defeat as well could be a serious setback to the government's ''reform'' program, and could even generate pressure for a general election, say some political analysts.

There are possible benefits to the National Party. A Treurnicht loss might well be the ''beginning of the end'' of the Conservative Party, says Hennie Kotze, a political scientist at Rand Afrikaans University.

However, Mr. Kotze says it is hard to escape the feeling that the by-elections are already something both the National Party and the Conservative Party regret.