Johannesburg — After nearly four years of verbal buildup, South African Prime Minister Pieter W. Botha has set sail on a clearly spelled out program of ''reform.''
The plan is radical in that it calls for the end to exclusive white rule. But it is also so limited in scope as to be potentially only a distraction from solving the central problem of enfranchising blacks in any meaningful way.
Mr. Botha's plan, unanimously approved by a rare federal congress of South Africa's ruling National Party July 31, is to open the central government to Colored (mixed race) and Indian minorities. Blacks, constituting over 70 percent of the population, would remain outside.
Mr. Botha had the ear of the nation when he spoke to the congress, but he chose primarily to try to unite his troubled National Party behind him. By supplanting the fiery ''adapt or die'' rhetoric of his early days in office with a call for ''practical politics,'' Botha appears to have succeeded.
The plan has met with criticism from all sides - but no outright rejection from any of the major parties concerned. There appears enough political leeway on all sides for continued debate among white, Colored, and Indian leaders.
Aside from opening the central government to multiracial representation, albeit in a way that ensures white dominance, the new blueprint calls for an overhaul of South Africa's Westminster-style system of government.
It would concentrate more power at the top in an executive president who would have a fixed five-year term of office. The appeal of this is the perceived need for a leader able to act boldly amid the mounting stresses in this racially divided society. The danger, say its critics, is that it is a prescription for a near dictatorship.
Shattering white exclusive rule is acceptable to the National Party not because of any change of heart over apartheid (enforced segregation), but because Coloreds and Indians have never had a neat niche in that ideology.
''Legitimacy is terribly important to the National Party,'' social scientist Lawrence Schlemmer says. And that legitimacy, he points out, is missing in the party's own eyes as long as the 2.5 million Coloreds and nearly 1 million Indians have no home in the apartheid structure.
''Coloreds and Indians occupy the same land as whites, . . .'' conceded Botha. Carving up ''homelands'' for them as has been done with blacks ''is not practical politics.''
A geographic separation of Colored and Indian is the alternative offered by the Conservative Party, which split from the National Party earlier this year on this question of ''power sharing.''
The National Party sees giving the vote to Coloreds and Indians as the necessary complement to a policy that would forever exclude black participation in the central government, Mr. Schlemmer says.
But if there is a whiff of fundamental reform in this proposal, it is that it sets in train a process with unknown results. Might it establish a model for working multiracialism?
Some say it could. The proposal both reinforces and moves away from strict racial segregation. The president would rule over three racially distinct parliamentary chambers, which reinforces separateness. Matters he deems of common interest would have to be approved by all three bodies.
But for the system to work, there will have to be a great deal of cooperation and compromise across the color line. Multiracial committees will try to achieve consensus on issues before they are put to votes. Further, when consensus is not reached, legislation will be turned over to a multiracial President's Council for a final decision that will be binding on the president. (The President's Council already exists in an advisory role.)
The president will be elected by a multiracial electoral college drawn from the chamber of parliament.
In the electoral college and in the President's Council white dominance will prevail. There is no suggestion of the National Party lessening its near-total domination of South Africa's political system.
But Pat Poovalingam, an Indian who quit the President's Council because it refused to address the question of blacks, says while the plan amounts to a ''very small step'' for South Africa, it is a ''giant leap for the Afrikaners.''