IN SOUTH AFRICA the confrontation over apartheid sharpens, and in the United States a feeling grows that Americans are doing too little for justice in South Africa and may even be comforting injustice. This is what is behind the rising impatience with the Reagan administration's policy of "constructive engagement" and, specifically, the countering campaign to legislate economic sanctions against South Africa. What is the right thing to do?

Above all else, the United States must speak in its true voice. Too often constructive engagement has amounted to a formula for pulling punches and making excuses for apartheid. While paying lip service to black dignity, American policy has often conveyed a sense that blacks are too impatient and that somehow whites are justified in resisting the black majority's demands. The burden should not be on blacks to show they deserve the rights they claim but on whites to stop denying equal rights to all South Africans.

Strong governments like the one in Pretoria bend chiefly to inexorable internal demands, which are mounting. In current circumstances, blacks are setting the pace of protest, and foreigners can do little to spare them the harsh immediate consequences (death, arrest, firing). But outsiders can assert, constantly and sharply, the standards South Africa ought to meet in response. Apartheid is an outrage, and neither whites nor blacks should be allowed to think the United States believes otherwise.

Sanctions express outrage, but that is at once their principal value and their principal deficiency. Easy slogans to the effect that Western investment and trade "finance apartheid" conceal the hard truth that sanctions would likely slow the engine of change that is the South African economy (or put firms in the hands of foreigners untroubled by apartheid), expose black job holders to direct loss, push white South Africa further toward an embattled self-reliance and punish South Africa's dependent black neighbors. The fallback argument, that sanctions, though of dubious economic effect, would have a considerable political shock value, stops working as sanctions move from the threatened to the real.

Americans should not let themselves be distracted by a debate on this second-order issue. The first order needs an unblurred focus: the steps whites should promptly undertake to end apartheid. The recourse to violence must go. The indecencies of discrimination must go. The pass laws and the group areas act, instruments for white control of black labor, must go. The homelands structure, robbing blacks of their common South African nationality, must go. The denial of black political rights must go.

In short, the alternative to sanctions need not be "constructive engagement," or doing nothing. It is targeting apartheid. Let the administration become as practiced in promoting the tasks of dismantling this odious system as it is in arguing against sanctions, and, we think, two things will happen: South Africans, who care deeply for foreign approval, will pay more attention, and the challenge to American policy at home will recede.