THERE IS light at the end of the tunnel of Angola's 15-year civil war. The government and its UNITA challengers have shredded the country and demonstrated that neither can defeat the other; they are engaged in direct negotiations. Their respective Soviet and American supporters are working in their fashions to turn them toward a cease-fire and political settlement in the coming months.

If this works, it will follow from a line of Reagan administration policy continued by President Bush. One early result was South Africa's delivery of independence to Namibia. A second was the departure -- completed in Namibia, close in Angola -- of Communist Cuba's expeditionary forces. The third would be a settlement in Angola. There American policy has sought not just to move out the Cubans but also to encourage Angolan national reconciliation and to persuade the Soviets that the price of their continued support for an unrepresentative local government is too high.

It is this last segment of American policy in Angola -- raising the price to Moscow -- that is now being debated in Congress. Everyone wants the Cubans to go home. Everybody wants a political settlement. But the Kremlin, disturbingly slow to apply its "new political thinking" in Angola, has kept immense aid flowing; its annual support, though down perhaps by a billion dollars from the 1987 peak, is put at $500 million. Meanwhile, some in Congress are opposed to or tired of sustaining Jonas Savimbi's UNITA, for which the Bush administration reportedly seeks $60 million. Voices in Congress demand that American aid be phased down or out. Some would use Rube Goldberg-type legislative instructions to try to arrange Soviet reciprocity, and some would simply end the aid.

Congress is right to be appalled that this awful war goes on. But a one-sided cutoff expresses only impatience and partiality for one Angolan party. It is not simply that the government in Luanda receives 10 times as much foreign assistance as the insurgents. Nor is it that either Angolan party has proven itself superior in dedication to the national welfare. Despite its relatively scant dimensions, American aid has served the crucial purpose of reinforcing the military stalemate and making possible negotiation aimed at a political settlement. To pull the plug on a policy that was failing might be understandable. To pull the plug on a southern Africa policy that has succeeded in other aspects and now seems near succeeding in Angola is perverse.