An American diplomat whose job includes making regular rounds in South Africa's black townships says she invariably hears the same message from the young activists she meets there.

"They complain that the United States is supporting the South African government," she recalled recently. "And they say we could make apartheid end if we would just use our power."

In a recent interview in his Pretoria office, the new deputy minister of information, Louis Nel, spoke from the other end of the political spectrum but expressed a surprisingly similar view of American influence.

Nel bemoaned the limited economic sanctions President Reagan has ordered against South Africa, saying the measures would cost jobs and set a bad precedent for other western nations. "America has the power to play a very positive role," he said with a note of longing and regret. "It could help create conditions for peaceful negotiations."

As these comments suggest, most South Africans still cherish a broad faith in the United States and its power to influence events here in spite of much disillusionment in recent years.

That belief stretches across the chasm separating white from black and supporters of the white-ruled government from its most ardent opponents. From the bleak townships of the Vaal Triangle south of Johannesburg to the wood-paneled government offices of Pretoria, political opponents who agree on little else generally still believe that the United States can play a major role here, although they differ sharply on what that role should be.

This belief finds expression from officials like Nel, who search for hopeful signs even as they condemn the new White House measures. "The action of President Reagan is wrong," said Nel. "But if I read what he's saying I get the impression he has the right instincts by and large."

Far to Nel's left, Anglican Bishop Desmond Tutu, the Nobel Peace Prize laureate, expressed a similar faith in American influence when asked in a recent interview if the United States could still make a difference in the outcome of events in this troubled country.

"Oh, it can, it very definitely can," Tutu replied. "Apply to South Africa the same policies you apply to Nicaragua and, voila, apartheid will end."

But these views of U.S. influence are being increasingly challenged, and not only by those on the left who see no positive role for Washington because of what they consider its silent support for the apartheid system of racial segregation.

It is also under attack from a growing number of white conservatives, for whom sanctions and the Reagan administration's increasingly vocal criticism of Pretoria indicate that even with a sympathetic president in the White House, America is too fickle and unreliable an ally to pin anything but false hopes on.

"We believe the United States should stop treating South Africa as a naughty child that deserves to be chastized and rebuked at every turn," complained an editorial last week in the right-wing Citizen newspaper.

"Like Rhodesia, Iran and other countries which bowed to U.S. pressure, we can only go one way if we listen to Washington. And that's down the drain," the editorial said.

That view is also reflected in the growing disenchantment among some whites with the Reagan policy of "constructive engagement" that when launched in 1981 found widespread support in the white community here.

South Africa's latest incursion into southern Angola and new charges that it has violated the 1984 Nkomati peace agreement with Mozambique have compounded white doubts about the policy, which had counted the fragile detente between Pretoria and its black neighbors as its main achievement.

"Constructive engagement in fact has been devised to serve the interests of the United States in southern Africa and not the interests of South Africa," said Carl Noffke, a former South African diplomat who now directs the Institute for American Studies at Rand Afrikaans University here.

"My opinion is we need to take a close look at the policy because I can't really name any part that has positively affected South Africa," he added.

Many of South Africa's white Afrikaners, proud of their frontier heritage and rugged independence, have long felt a special affinity for the United States -- or at least for their idealized image of what the United States symbolizes.

There also has been a certain ambivalence: while craving American respect and sympathy for their problems, Afrikaner governments often have reacted to American criticism by reaffirming their sovereignty and their ability to exist in international isolation if necessary.

Suspicion of American promises and motives also runs deep and dates back in part to 1975, when the Ford administration first gave tacit encouragement to a South African invasion of Angola, then withdrew its support when the operation encountered strong international criticism.

It is said here that President Pieter W. Botha, then South Africa's defense minister, has never fully trusted American promises since then.

President Reagan has remained deeply popular among whites despite the misgivings about his administration's policies. His off-the-cuff comments that appeared to justify the shooting of black protesters by security forces and claiming that much of the country's segregation has been abolished have been cheered by many whites here who still believe they have a friend in the White House.

"He's the only sane element left in American foreign policy," said Noffke.

But the sanctions Reagan announced two weeks ago have stirred all of the old fears of isolation from the West that haunt whites.

Many fear that the measures, combined with the refusal of western banks to make new loans, are the first steps in launching South Africa down a path to economic disaster and, ultimately, black revolution.

Some analysts say the South Africans have themselves to blame for not taking better advantage of Reagan's policy to enact reforms that might have satisfied moderate critics internally and in the West. Botha's two-month-old emergency decree, the arrests of more than 3,000 opponents and the harsh police crackdown on public protests are all indications that the government is not responsive to friendly diplomatic persuasion, according to these observers.

"The government believes America is fickle," said political scientist David Welsh. "But if you ask what they've done to keep constructive engagement in business, the answer is absolutely nothing."

Across South Africa's rigid color line, there is also increasing disenchantment with America among blacks and a growing radicalism that links apartheid with western capitalism and its leading proponent, the United States.

One measure of this is the growing anti-Americanism openly expressed at political funerals. "Reagan is a terrorist," said a young anonymous speaker at a funeral for 18 blacks three weeks ago in Duncan Village, outside East London.

Then, rattling off a string of emotional accusations, he charged, "Sixty-five percent of all the wealth of this country is going to America. The American government is manufacturing computers to kill our people."

Just as whites generally admire Ronald Reagan, blacks here generally distrust and despair of him.

Bishop Tutu's increasingly stinging denunciations of the president as a "racist" find resonance here, although many activists contend that Tutu's appeals for a change of heart by the administration are naive and too late.

"Tutu has become a voice in the wilderness," said Saths Cooper, vice president of the Azanian People's Organization, one of the radical groups that opposed Sen. Edward M. Kennedy's visit here in January and that claim to have little use for American sympathy or support.

"People believe in their own ability to effect change, not in what outsiders can or cannot do. We are way past that point," he said.

But Prof. Lawrence Schlemmer, a social scientist who has done extensive polling on black attitudes, contends that Cooper's claim does not reflect the black majority. "It's actually one of the weaknesses in internal black politics here that people still have too much faith in what the external world can do," said Schlemmer.

"There's still an intense, even naive hope that the United States will come along and solve problems, he said.

"It may be true that the black intellectual is disenchanted, but for the average black person America remains the land of Coke, Kennedy, Superman and all manner of wonderful things."