The president's national security affairs adviser, Robert C. McFarlane, answering a question about South Africa on "This Week with David Brinkley," said, with a perfectly straight face, "the president believes it is not up to the U.S. to prescribe what is right for other people."

Is that so? In Nicaragua, it certainly isn't. The United States is prescribing nonstop what is right for the people of Nicaragua, giving active support to the contras trying to overthrow the government.

Typically, the administration denies that overthrow is its intent. "Absolute nonsense," says McFarlane of the obvious.

Nicaragua can never hope for the forbearance, indulgence, sympathy and understanding enjoyed by South Africa under President Reagan's program of "constructive engagement." Nicaragua has been subjected to economic sanctions.

What is clearer every day is that, if the president were to switch his policies and apply "constructive engagement" to Nicaragua and economic sanctions to South Africa, he would make himself and the United States look good. He would restore America's position as the moral leader against apartheid and, in Central America, claim some semblance of the statesmanship that would become the world's largest democracy better than bullying a small leftist, dirt-poor country into saying "uncle."

Intervention has brought death to Nicaragua. Intervention could reverse the bloody tide of events in South Africa.

Bishop Desmond Tutu, the Nobel prize winner who has put the case of his country and his people with incomparable eloquence and passion, says South Africa is "on the brink of disaster unless America intervenes."

"Constructive engagement" was always tenuous. It gave Reagan the excuse to embrace the white South African government, while condemning from time to time the apartheid it practices. Now the policy has become a sick joke. President Pieter W. Botha made a churlish, mean-minded speech last week in which he promised to give blacks citizenship without the vote and told the rest of the world to go to hell.

It was a massive insult to Reagan. Botha had trumpeted the speech as a major breakthrough. But Reagan came back smiling, still for "constructive engagement," still against sanctions.

Economic sanctions would be ideal for South Africa. They would not cause unendurable hardship but would serve to make the point that the United States will not tolerate apartheid, no matter how anticommunist the government may be. Pious claims from the administration that the real victims would be blacks, who would lose their jobs in a cutoff of U.S. investment, have only the ring of howling hypocrisy.

Botha knows that, being anticommunist, he is safe.

On the other hand, Nicaraguan President Daniel Ortega, just as clumsy as the president of South Africa, is scared stiff of the United States and anxious to make friends. Ortega, a Castro groupie, sent Reagan Fourth of July congratulations and a get-well message during the his illness. Ortega's thanks was to have Reagan put Nicaragua on his list of terrorist nations.

Reagan has all the time in the world to talk to the white government of South Africa, none to talk with Nicaragua.

Botha will talk to Tutu only in the presence of other religious leaders. Tutu, who sees his people being whipped in the street, who fears they will turn to apostles of violence, says this would be a waste of time.

You might think that the administration would back the charismatic bishop's appeal for a one-on-one meeting with Botha. Instead, the president's men, anonymously, are criticizing the bishop for not attending a general gathering of religious leaders with Botha.

The president thinks everything is up to him in Nicaragua. Military force and economic pressure must be applied. The rebels must be taken into the government. He has assigned a member of the White House staff, Lt. Col. Oliver North, to help raise money and plot strategy for the contras. The Nicaraguans have to be punished for what they might do if he let up.

Elliott Abrams, the State Department's chief for Inter-American affairs, explained why simply reaching agreement with the Sandinistas would not be enough: "If, say, three months after such an agreement the contras were disbanded, the guerrillas would begin to reinfiltrate into El Salvador within a month, where would we be?"

So we meddle mortally in Nicaragua and defer fatally in South Africa. A swap is strongly indicated; if that is not possible, a little consistency, at least, would be appreciated.