IF IT IS going to be truly constructive about it, the administration should engage itself in South Africa with the African National Congress, the black nationalist organization forced underground by the apartheid regime and one that appears to speak for many South African blacks. The administration, it is true, has urged the South African government to release the ANC's long-imprisoned leader, Nelson Mandela, and open with him and other representative blacks a broad political negotiation. But it has been excessively wary about approaching the ANC itself. Its contacts with the ANC have been on a low level and, it seems, few and far between.

The South African government points to the communist associations of some ANC leaders and the military and other support provided by Moscow and depicts the ANC simply as a creature of the Kremlin. Most knowledgeable people find this a crude portrait that overlooks the origins and content of the ANC's ties to Moscow. To the extent that it is true, it is an argument not for stiffing the ANC but for competing to influence it, especially those elements in it that resist and diasapprove of the organization's growing tendency toward terrorist acts -- the burning to death of blacks accused of collaboration with Pretoria and the bombing of "soft" white targets.

Considerations of the sort that keep Washington at a distance from the ANC prevailed in Britain until very recently. Then, as pressure rose on London to respond with new sanctions to South Africa's latest outrages, a government minister received ANC leader Oliver Tambo -- a step apparently conceived as something of a substitute for sanctions.

Ronald Reagan shares Margaret Thatcher's hesitations about sanctions. His administration, in an election year, is also under pressure to support tougher ones. Opening up to the ANC is valuable and necessary for its own sake, a course that an administration attempting to promote peaceable discourse should have no difficulties in reinforcing by its own example. But opening up has another advantage for the Reagan team. It shows an understanding of the black cause, and it could be used to deflate sanctions pressure. The administration has been debating the ANC option for some time and may be edging that way. It is the right way to go.