RONALD REAGAN had his chance to lead American policy on South Africa at a crucial moment when the United States was looking for a way to help turn civil war to conciliation. By a speech so pinched and equivocal that it drew cheers only in Pretoria, he forfeited that chance, and now the Senate is moving into the gap. Several tendencies are in evidence there, one leaning toward the across-the-board sanctions that the House startled itself by voting last month and a second effort, led by Republicans Lugar, Dole and Kassebaum, looking toward a more selective approach.

We think the Republican moderates are on the right track. They realize that the United States must be and must be seen to be enthusiastically and forthrightly on the side of black freedom rather than white privilege. This is not just a matter of posturing for political effect, although it is an important political development -- one that we hope is noted in Pretoria -- that active support for the end of apartheid is now widely seen as an electoral imperative. America is a multiracial society that strives to ensure equal rights for all its citizens, and this impresses a moral stance on American policy toward South Africa. With Americans now stirred as never before to the horrors of apartheid, a policy that ignores this reality can only fail of popular support.

The Republican senators have a sensible view of sanctions. It is that their effects cannot be precisely calculated, and it is surely unwise to expect, as some supporters of the House bill do, that the apartheid regime's grip on power is so slight and uncertain that it will inevitably loosen as Western sanctions are applied. But where persuasion has produced inadequate results and the situation within South Africa is sharply deteriorating, it is necessary to try turning up the pressure. The operating hope of the moderates is to alter the internal equation in South Africa -- to give a stronger hand to those who feel it is in their country's urgent interest to soften the regime's continuing resistance to prompt, honest negotiations on a new political order. Hence the Lugar bill provides for new sanctions, targeted as much as possible on the favored whites, but offers the regime relief if it turns to the political path.

Argument over the wisdom and effectiveness of sanctions will go on. The next few weeks and months, however, are an interesting time to conduct it. The South Africans can see both the United States and Europe gathering for a leap forward on sanctions. It is the right time for the sort of political concessions that will moot sanctions and put South Africa on a path to justice.