When F. W. de Klerk meets with President Bush tomorrow, it will mark the end of a long and troubled era in U.S.-South African relations. That Bush is willing, even eager, to receive de Klerk is evidence of the changes that have occurred in South Africa over the past year and, just as important, of the shifts that have occurred in American attitudes toward the South African government.

Not since the 1940s has a South African head of state visited the United States.

The official welcome de Klerk will receive in Washington is a reward for his Feb. 2 move to free Nelson Mandela, un-ban the African National Congress and other anti-apartheid organizations and begin to negotiate an end to white-minority rule. Beyond providing an encouraging pat on the back, it is not clear that the United States can do much to help de Klerk address the challenges facing his government.

On Aug. 6 de Klerk and Mandela reached agreement on a series of issues including the release of political prisoners and the suspension of the armed struggle. The next step is to negotiate the process that will be used to draw up a new constitution. That is not likely to prove easy.

So far, de Klerk has chosen to negotiate almost entirely with Mandela and the ANC. This decision made sense given that the most important obstacles that had to be surmounted before negotiations could begin involved conflicts between the government and the ANC. But it has also created fears among the ANC's rivals, especially Chief Mangosuthu Buthelezi and his Inkatha Party, that they will be denied a political role in a post-apartheid South Africa. These fears are partly responsible for recent attacks on ANC supporters in black townships in the Transvaal.

For the negotiations to move forward, de Klerk (and Mandela) will have to convince supporters of parties that have yet to be given a seat at the negotiating table that they will not be left out in the cold.

A second challenge to de Klerk comes from within the white community. Despite the results of last September's elections, when over two-thirds of white South Africans voted for leaders committed to change, it is doubtful that a majority of whites are willing to support negotiations that might result in the election of an ANC-led government. De Klerk is gambling that he will be able to bring whites along over time. But the Conservative Party and other right-wing political parties are already attempting to use the recent violence to mobilize political opposition to his policies.

Of greater immediate danger is the possibility that white extremists will use violence to disrupt negotiations. Such groups probably have had a hand in the recent violence in black townships. The damage that these groups can do is increased by political divisions in the townships and the existence of a large pool of brutalized, unemployed blacks -- both of which create a ready supply of recruits for right-wing provocateurs.

In dealing with threats from the right, de Klerk must walk several political tightropes. He must negotiate away the white monopoly on political power, while reassuring his white constituents that their welfare and security will be protected in a post-apartheid South Africa. He must crack down on continuing human rights abuses by elements within the police and security forces but at the same time not do anything that would precipitate a revolt within the ranks of those forces. The decision to prosecute Winnie Mandela for her role in an assault on several black youths could be a response to these pressures. It is possible that de Klerk and his advisers felt it necessary to prosecute her in order to preempt a conservative backlash against actions to control renegade policemen and crackdown on Inkatha.

Finally, and most fundamentally, de Klerk faces the challenge of reconciling his government's vision of a post-apartheid South Africa with black visions of the future. A wide gap still exists between the government and most blacks on issues such as the powers of a national government, the rules by which political representatives will be elected, the constitutional protections that will be accorded groups as opposed to individuals and the structure of a future economy.

Can the United States help de Klerk? Probably not. He and his advisers understand the challenges they face far better than anybody in the White House or the State Department. Moreover, with the exception of Buthelezi, we are not in a position to influence the groups -- e.g., extremist whites and militant blacks -- who are creating the most problems for de Klerk. And given our budget problems and preoccupations with the Persian Gulf, it is doubtful that we have the resources and attention span that would be required to play a leading role in addressing South Africa's problems.

In these circumstances, our most important objective should be to avoid exacerbating de Klerk's problems. For this reason, the Bush administration and Congress must resist the understandable urge to reward him by lifting sanctions.

It is time to begin lifting sanctions. But this must be done as part of a comprehensive international effort to create the economic base that will be required for the emergence of a stable democratic South Africa; and it must be done in cooperation with Nelson Mandela and other anti-apartheid leaders.

If sanctions are lifted in any other way, the political base of moderates within the anti-apartheid movements, especially Mandela, could be seriously damaged. And that would make all of the problems de Klerk faces far worse.

Michael Clough, who was study director of the Secretary of State's Advisory Committee on South Africa in 1986 and 1987, is a senior fellow at the Council on Foreign Relations. Nomsa Daniels is a staff assistant at the council.