IN THE BIG anti-apartheid legislation of 1986, Congress imposed sanctions against South Africa but still made a powerful argument for a full place for blacks in the country's economy. The bill said Congress ''applauds'' American corporations that follow equal-opportunity codes. Though President Reagan vetoed the bill for its sanctions, then and since he has joined Congress in urging an American business presence sensitive to black needs. This was the central shared element of a policy otherwise in hot dispute.

No more. Congress has lost patience with the view that there is a useful purpose to be served by staying on. More pressure is its policy now. This is how the Rangel Amendment came to be tucked quietly into the budget reconciliation act. Introduced by Rep. Charles Rangel (D-N.Y.) in the House, the measure was accepted unanimously by Senate conferees; the State Department protested it, but it did not make the short list of items the president had said he would veto the whole bill to block. The amendment repeals tax credits that American firms receive in the United States for taxes they pay to South Africa. In effect, it subjects to double taxation the 150 or more American firms still doing business there, and it will increase measurably the likelihood that they will go home.

It is a mistake. These are firms in the vanguard of equal rights, and they will sell out -- some of them, no doubt, at distress prices -- to white investors who will be under no similar pressures. The jobs and professional openings of their black employees will fall into fresh jeopardy. No less vulnerable will be the black trade unions -- leaders in the struggle for black empowerment -- which those employees support.

An unhappy confusion about fighting apartheid is evident. The right way for the United States to help is to do whatever it can to quicken peaceful change. This is where staying -- yes -- constructively engaged to serve black economic advancement fits in. The wrong way is to yield to a generalized despair and militancy that fail to respect the real effects of particular measures. In this instance, the step taken undercuts firms that met Congress' own good and still-sensible equal-opportunity standard. In the name of ''pressure,'' it stands to contribute to misery, bitterness and the explosive potential of a country that needs no new gasoline thrown on its fires.