THE ESCALATING demonstrations against the Reagan administration's South African policy and the administration's defense of itself share this one assumption: That Washington can do something to compel the South African government to change its apartheid policy. But despite the moral fervor on one side and the pleas for pragmatic moderation on the other, there is ample evidence to suggest that almost nothing done in Washington -- neither the Reagan administration's policy nor any of the suggestions for more "pressure" -- will have much influence on what happens in Pretoria.

That suggestion flies in the face of policy discussion in Washington about how to induce change in South Africa. For four years, Chester A. Crocker, the Reagan administration's assistant secretary of state for African affairs and architect of its "constructive engagement" policy, has been touting a no- confrontation approach toward South Africa. Crocker has been telling Americans that South Africa is changing, that slowly but surely its white rulers are moving, however reluctantly, toward a system that would allow genuine participation by the country's 73 percent black majority.

Recent events have made that claim look hollow. Within the last three months, angry black protests and a harsh South African security crackdown in response have resulted in more than 130 deaths by the official count and the detention without trial of perhaps 1,000 persons. The "new" South Africa, with its 90-day detention laws, its predawn police roundups and its strictures against free speech and movement, is looking very much like the South Africa of old.

Likewise, the incremental changes of recent years -- legal recognition for black trade unions, increased spending on black education and, most recently, implementation of a new constitution granting limited right to brown, but not black, South Africans -- appear far overshadowed by the actions of a state that retains and is willing to use all of its wide and relentless power against those demanding equal rights.

The result has exposed what critics contend is a fatal flaw in administration policy. For Crocker's strategy is based upon the premise that the South African leadership is amenable to peaceful change, and that friendly American actions encouraging such change make more sense than the hostile policy pushed by the Carter administration, which may have felt good morally but only succeeded in alienating further the ruling white Afrikaners and rendering them more obdurate.

The recent arrests of members of Congress and others at the South African embassy in Washington forced Crocker this week to make an unusual appearance before White House reporters to answer criticisms of administration policy. In his spirited defense, Crocker suggested that constructive engagement may even have been a factor in persuading South African President Pieter W. Botha to proceed with the limited reforms that led conservative rightist Afrikaners to split from his ruling party in 1982.

Critics of Crocker argue that his policy has had just the opposite impact of what he intended, that excessive American sympathy and the lack of criticism has been interpreted by the South Africans as giving them free rein to do what they want both at home and in the region.

But while Crocker and his critics argue over what Washington should be doing, South Africa's leaders and their opponents are reacting to other, more pressing imperatives: At home, the interplay of divisions within white Afrikanerdom and the growing economic potential of blacks; in the region, the need perceived by Botha and his military strategists to create a cordon sanitaire of black neighbors who are docile, if not ideologically compatible.

This much was made clear to me in an interview earlier this year with Deputy Foreign Minister Louis Nel. Nel is one member of the new generation of upcoming Afrikaner leaders who recognize the need for change and want continued American support. He spoke glowingly of Crocker and disparagingly of the Carter adminstration, saying Crocker had made great progress in bringing about a regional detente between Pretoria and its Marxist neigbors, whereas Carter's men had gotten nowhere. America, he said, again had influence because it was playing "a positive role"

Yet when the subject of the recent eviction of 300 black villagers from a designated "white area" in the northen Transvaal came up, America's influence suddenly vanished. An unusual public protest by the State Department had helped delay the move, but could not kill it. Nel was quite clear that this type of issue, which lies at the very heart of the apartheid system, was "none of your business."

Nel concluded with a comment that is so obvious it would not bear repeating had it not been overlooked so often in the debate between Crocker and his critics. "We will listen to any friend, " he said. "but at the end of the day, South Africa will make its decisions on what it judges to be in the best interests of its own people."

Even the example cited by Crocker -- Botha's push for limited constitutional changes -- bears Nel out. The split between Afrikanerdom's center and its right had been building for at least a dozen years before Crocker came along. The split occurred when it did because P. W. Botha, who is among other things a canny, grassroots machine politico who would feel well at home in Mayor Daley's Chicago, calculated that he needed to expand his constituency and that he could afford to part with his already disaffected right wing. Constructive engagement, as Chester Crocker might well concede in a quieter moment, had nothing to do with it.

Although America's levers may be few, its concern about the fate of South Africa is real and significant. Part of the reason is that we have a lot of money tied up there: at least $2.6 billion in direct investment and possibly as much as $15 billion when all forms of holdings -- including loans, stock shares and investments made through European subsidiaries of American corporations -- are added up.

We also have a lot of moral capital invested in South Africa. Part of that is old- fashioned human concern over the last nation on earth where racial domination is enshrined in law. And part of it is a peculiar American fascination with a country whose racial problems reflect, albeit in a distorted and substantially different fashion, our own.

So we care. But translating that concern into a coherent policy has never been easy. For one thing, there is no easy way to pressure South Africa. It receives no military or other aid from the United States. Its economy, while slumping, is relatively autonomous because of its mineral wealth. Its ruling Afrikaners are on the periphery of western culture and values: They tend to identify themselves as Africans as much as Europeans. They are fiercely proud and independent, more inclined to go it alone in a hostile world than to compromise on what they consider to be their future survival.

Then, too, U.S. policy has always been limited by certain assumptions. One is that violence is counterproductive and always to be avoided. A second is that American investment and capitalism in general have been forces for social and political change inside South Africa and thus, although they ought not actually be encouraged, they should not be hindered either. Democratic presidential candidates often speak about imposing economic sanctions against South Africa if it refuses to cooperate. Democratic administrations, like Republican ones, have never done so.

As Bishop Tutu likes to point out, neither of these assumptions is unchallengeable and both have been spurned by the United States in dealing with other governments whose policies we claim to abhor. The Reagan administration sought to apply the full weight of economic sactions, albeit without great success, against martial law in Poland. The idea that armed revolution is a legitimate tool to fight an unjust government is surely one of the operating principles behind administration policy toward Nicaragua. None theless, the Reagan administration does not give arms to the African National Congress, the major South African resistance movement. Nor did Jimmy Carter.

We are, in short, unwilling to blast P.W. Botha from his chair or to cut off his oil. As a result, having rejected using military or economic tools that could do the most damage to apartheid, we are left to nibble at its edges. This has been the operative reality of U.S. policy, whether devised by Chester Crocker or Andrew Young.

In the end, much of what constructive engagement has done fits well into the "nibbling" category. The heady, early days when then Secretary of State Alexander Haig could propose a public toast hailing South Africa and the United States as "old friends . . . who are getting together again" have passed. These days State Department officials and President Reagan himself are more inclined to emphasize how much they dislike apartheid in phrases that don't sound all that different from those of Jimmy Carter.

Beyond quiet diplomacy, Crocker's most obvious innovation has been to drop or ease restrictions on the sale of strategic goods. According to a recent survey by the weekly Africa News, U.S. government-issued licenses for sensitive items much as aircraft, computers and communications equipment have increased nearly 100 percent this past year, while licenses for non-lethal, military- related equipment on the government's munitions list have risen from a previous $12 million to $88 million.

All this is consistent with Crocker's argument that there is no point in "erecting foolish pinpricks" that will only further anger the South Africans and could constitute the beginning of economic sanctions "through the back door of incremental restrictions." But this position looks highly vulnerable, and will be a target of opportunity for critics, in the light of the latest police crackdown.

Otherwise, the ledger on constructive engagement is uncertain and somewhat blank. Domestically, for the reasons mentioned, it seems to have had little impact. As longtime government critic Helen Suzman, an opposition member of parliament, points out, the determining factor inside South Africa has been one that Americans have had almost no role in: "the steady upward movment into skilled occupations by blacks eventually giving blacks the muscle with which to make demands for shifts in power and privilege, backed up by the force of black urbanization."

Regionally, Crocker can claim credit for helping broker a nonaggression pact between South Africa and Mozambique and a disengagement accord with Angola that could be an important step toward an independence settlement in Namibia. But it can be justifiably argued that these achievements are mostly the result of hard- nosed military tactics by Pretoria that coerced recalcitrant but weak neighbors to the bargaining table and that Crocker at best helped ease the process.

The recent spurt of optimism over a Namibian settlement and Cuban troop withdrawal from Angola does not disguise the fact that, so far, little has been accomplished. The critics are probably right to contend that Crocker's strategy linking Namibian independence to Cuban withdrawal has given Pretoria a convenient excuse to delay. But the South Africans would have come up with another excuse had they needed one, for they are likely only to move from Namibia when they perceive it is in their best interests to do so.

The critics are certainly right about one other point. Constructive engagement has alienated a significant segment of the black community, both within South Africa and on the rest of the subcontinent. Fairly or not, American policy is identified as one of moral support for white rule. South Africa is seen as serving the role of stand-in for U.S. interests in the region. As much as Crocker tries to play the role of honest broker, it is hard not to notice that the two southern African leaders most critical of the Washington- Pretoria connection, Zimbabwe's Robert Mugabe and Tanzania's Julius Nyerere, have been the two whose American aid allocations have been most sharply cut recently.

Perhaps the real problem with American policy toward South Africa has little to do with constructive engagement and more to do with traditional American notions of optimism in analyzing the affairs of other nations. Americans tend to take the view that what is lacking in South Africa is better communications between the races and the willingness to sit down and negotiate. If we can only get the two sides to begin talking, this reasoning goes, good things can happen.

Typical of this optimism has been the view that capitalism, by creating a need for skilled and mobile black labor, is helping tear down the walls of apartheid peacefully. But comforting as this seems, there is little historical data to support it. Capitalism and apartheid have been coexisting, however uneasily, for 36 years -- since the Nationalist Party came to power -- and both seem to be more flexible and accommodating than their theorists would allow. To ask urban blacks to stand by peacefully until capitalism frees them means asking them to wait 50 years. To ask rural blacks to do so means their permanent consignment to the scrap heaps South Africa calls "homelands."

Few South Africans subscribe to American optimism. They tend to see politics as a zero-sum game -- when others gain, they lose. Thus white Afrikaners may cede certain concessions, possibly even the vote, to limited numbers of blacks. But they will not cede control. Similarly, blacks may settle for nothing less and these two groups will continue to clash.

A crisis will come to South Africa. It may come tomorrow, but it is more likely to come in 10 years or 20 or 30. When it does, Washington will have an important role to play in easing the process of change and in lessening the violence, or bringing it to as speedy a conclusion as white resistance and black ambition allows. It will be the same role the United States played along with Britain in the birth of Zimbabwe. Until that time, so long as Americans cling to their basic assumptions, gradualist policies such as constructive engagement are likely to matter little.