A FANTASY is overtaking American thinking about Angola. It is that the United States can go back into the business of supporting Jonas Savimbi's UNITA rebel forces -- through a door opened by congressional repeal of a 10-year ban on such support -- and suffer no untoward consequences. Mr. Savimbi's admirers in the administration and Congress are acting as though the strategic and ideological rewards of aiding this African "freedom fighter" are there for the picking.

Mr. Savimbi is certainly somebody in Angola, although those who know of his earlier incarnation as a Maoist may wonder about his more recent debut as a democrat. As a tribal leader, he has shown military and political staying power. No less than the rival tribal leader who is the country's Marxist president, he can claim to deserve a place in Angola's future.

But that's not the whole of it. Mr. Savimbi is South Africa's man in Angola. He takes apartheid's support reluctantly and only for his own goals, he insists, but he takes it. That makes any foreign backer at least an implicit partner of South Africa. To ask Africans to overlook this link, or to explain it to them as a tactical alliance that the United States enters for necessary purposes of curbing Soviet power, is absurd.

Enthusiasm for a cause that it was uncertain the administration embraced led Congress to try to force the administration's hand by offering the Savimbi forces public aid. Now officials seek to tuck new aid discreetly under CIA sponsorship. American diplomats hope Angola will be sobered just to see the aid being discussed, and they have resumed their effort to negotiate Cubans out of Angola and South Africans out of Namibia, together.

It is necessary to be clear, however, about just who was responsible for the impasse in that negotiating effort. It was considerably more South Africa than Angola. And that is what is so troubling about the idea of new aid for Jonas Savimbi. The country that tended to cooperate with Washington is being "rewarded" with the threat of American support to an internal challenger. The country that defied Washington -- stonewalled in the talks on Angola/Namibia, set out to sabotage a big American oil installation in Angola -- is being "penalized" by the offer of an implicit American alliance of tremendous strategic and political value. This is diplomacy?

Mr. Savimbi, as we say, has a claim to a place in the Angolan sun. But his chances of getting it seem to us to diminish if the United States intervenes in the Angolan civil war in his behalf. Large scale aid -- enough to replace South Africa's -- would have to match a Soviet commitment that has already brought Angola several billion dollars' worth of assistance. Anything less would only hurt the United States politically without helping Mr. Savimbi much militarily. The better way is to try to revive the negotiations that were dragging, but not dead, earlier this year.