THE REAGAN administration is bumping against the limits of its ideology in southern Africa. The true enemy, according to its latest policy pronouncements is "revolutionary violence" of the Marxist or Soviet-supported sort. Yet to most Africans, and not only to them, "revolutionary violence" is mainly the label that South Africa and supporters of its ways apply to those who challenge the power wielded inside and outside its borders by South Africa's white minority government. Inside its borders the Pretoria regime has lately been shrinking the space available for peaceful change. Outside, especially in the Namibia-Angola sector, it has been freely applying its great advantage in conventional power. In word and deed, the Reagan administration appears to be going along.

The Namibian crisis is becoming especially acute. South Africa, seeing Ronald Reagan coming, slipped past the international effort led by Jimmy Carter to grant independence to the colony it took over from the Kaiser's Germany after World War I. That left it squarely up to President Reagan to use his conservative bona fides to coax South Africa to let Namibia go. The administration has been extremely solicitous in caring for Pretoria's security and political interests in Namibia. Pretoria, however, gobbles up every concession and indulgence and invites more. It acts as though Mr. Reagan prizes its anticommunism and its economic assets so highly that he will not really object if it grants a mock independence to a Namibian puppet regime. The administration contends that progress is being made, but the unanswered question remains whether Mr. Reagan is being gulled by the South Africans or whether he is winking at them.

This is the backdrop to the fighting across the Namibian-Angolan border.Angola provides sanctuary to the SWAPO guerrillas contesting the Pretoria-backed coalition in Namibia. South Africa regularly strikes back -- to pursue the guerrillas and, not so incidentally, to punish Angola for harboring them. After the latest such raid, an especially long, deep and destructive one in which Soviet supporting forces were also engaged, a resolution was introduced into the Security Council condemning South Africa. Alone, the United States vetoes it on grounds that in singling out Pretoria's attack and ignoring SWAPO's provocation the U.N.'s Third World-communist majority was doing its familiar unfair thing.

It is true that the United Nations, by dozens of votes like this over the years, has shredded its claim on the attention of serious people. Yet the Reagan administration gives the impression of shielding its own real policy confusions behind the U.N.'s institutional flaws. The heart of the matter is that the president has not yet made it unequivocally clear that South Africa will not be allowed to bo back on its word to grant independence to Namibia. South Africa is the occupying power there.It has no right to be there. The very hint of American collusion to enable a regime Africans detest to harden its grip on a fellow African people undercuts American interests in the region.

The administration keeps insisting that "progress toward a Namibia settlement could set the stage for withdrawal of Cuban forces from Angola." Fine. But does it not then follow that the administration should be using its influence to deliver South Africa to a settlement? In fact, the Reagan team wants much more in Angola than the departure of the foreign communist forces protecting and supporting the Luanda regime. It demands that Luanda somehow make a place for its challenger, the "significant and legitimate" -- and anti-communist -- Jonas Savimbi. The many governments that recognize Luanda consider that the Angolan civil war ended in 1976. The American government holds that it is still on.

This creates the administration's dilemma. Mr. Savimbi has a claim to being, among other things, a nationalist with a demonstrated constituency and geographical base. Yet to support him is to fall into a partnership with South Africa that could cripple American dealings elsehwere in Africa, and it is also to give Luanda yet another prtext -- Savimbi as well as SWAPO -- to cling to the foreign communist whose presence most troubles the administration in the first instance. In short, the chief obstacle to the administration's effectiveness in southern Africa is itself.