ALREADY the argument against the protests at the South African Embassy is getting into gear. Yes, it begins, apartheid in white-ruled South Africa is reprehensible. But in black Africa many terrible things have taken place: at the moment, tribal rivalries and politics are impeding food relief, and people are starving to death as a result. How can American blacks who are demonstrating against the South African regime expect to sway the public's moral sensibilities when they, or most of them, have not established their credibility as moral witnesses by protesting against the policies of those particular black regimes that create monstrous mass suffering of varying black populations?

Where it is not being made simply to advance a narrow reactionary agenda or to apologize for apartheid, the argument has a certain impact. Certainly it would be a mistake for sponsors of the new demonstrations to evade it. Otherwise, they risk having the cutting moral edge of what they intend to be a lasting and growing political movement -- something new in American politics -- dulled right at the start.

Some would finesse the charge of moral arbitrariness by asserting that apartheid is a unique evil -- alone in the world, South Africa has put the apparatus of a modern industrial society in the service of perpetuating a despicable organized racism -- and that an attempt to inhibit an assault on it is itself morally objectionable. But it is no defense of apartheid to observe that its moral squalor does not depend on its being unique in one particular way or another and that the case against it surely does not weaken by acknowledgment of the fact that other terrible tyrannies exist in the world in general, including some on the African continent.

It is further argued in defense of making the assault on apartheid the only such campaign that choices are often made on a political basis among competing or at least different objects of compassion. There is not much of a lobby in this country, for example, for a state for the Kurds. So why are not blacks entitled to lobby for a policy of their choice in respect to South Africa, without being asked to apply a single standard to black and white African regimes? If they go that way, their influence on policy may not then extend much beyond their already limited political weight. But we think the opposite is true: A greater influence could be asserted if blacks and those supporting the pressure on South Africa (we are enthusiastically among them) would broaden the moral base of their appeal.

In the past, too few American blacks addressing African issues have been ready to speak out strongly against all injuries done to blacks, whether perpetrated by blacks or whites. That sort of selectivity should end. A requirement for a single moral standard may be difficult to accept, but it is the essential condition for building a broad-based anti- apartheid coalition reasonably safe from the left- right, liberal-conservative wars. Only such a coalition will last and, lasting, make a difference.