The temptation now lying before the Reagan administration is to resume aid, military or "humanitarian," openly or secretly, to insurgents fighting in Angola. The repeal of a 10-year-old ban on such aid last summer opened the door and, The Post's David Ottaway reports, the administration is considering whether to walk through.

It is, surely, a powerful temptation. The Angolan government inclines to Marxism, relies on Soviet- bloc support, including Cuban troops, and offers Moscow a presence in a strategic African quarter. Jonas Savimbi's UNITA challenge has nationalist credentials not inferior to the governing MPLA's, a democratic face and a decade-long record of perseverance in the field. The Reagan team had hoped to negotiate Cuba's withdrawal from Angola and South Africa's from neighboring Namibia, but that effort is stalemated. The State Department is under pressure from conservatives elsewhere in Washington, in and out of the administration, to apply the "Reagan Doctrine" of support for anti-communist freedom fighters. It would, they say, have the bonus effect of sending Moscow a signal of resolve.

But all this is making policy with blinders on. One source of the 1976 aid ban was the post-Vietnam backlash against whatever smacked of "involvement" or "intervention," especially anything incremental or covert. That has diminished in public urgency -- look at American aid to the resistance in Nicaragua and Afghanistan -- but a second source of the 1976 ban has not. We refer to the judgment that it would be damaging and wrong to become a regional partner of South Africa, Mr. Savimbi's leading foreign sponsor.

The intent to keep at a distance from Pretoria was sound in the mid-1970s. It is sounder in the mid- 1980s. American politics is more seized of the issue of South Africa and the foreign-policy stakes in Africa are higher.

Anyway, the United States already has a policy. It hasn't yet worked but, we think, it still carries more promise and less risk than a new course. It's the effort to broker a South African-Angolan accommodation and in so doing to see UNITA -- still to the American government "an authentic African nationalist movement" -- find a political place in Angola. Currently the Washington-Luanda lines are down, but the Angolan government seems interested in reopening them. The South Africans are cool, but even if they were warm it defies good sense that the United States could usefully join them in this project.

The term the administration has applied to its southern Africa policy, "constructive engagement," has no takers anymore, but the idea of a helpful American diplomatic role remains relevant. The alternative is not support for the insurgency; it's leaving the field to Moscow.