JOHANNESBURG, SEPT. 20 -- Negativism among black leaders toward offers to negotiate political change in South Africa could postpone black-white dialogue on sharing power, according to the government's point man on reform efforts.

Citing his fear of an "automatic negative syndrome" similar to one he said that is displayed by Palestinian leaders in the Middle East stalemate, Stoffel van der Merwe, deputy minister for constitutional planning, said it is urgent that the government tries to penetrate that mistrust.

"Sometimes I get the impression that people on the other side are so committed to saying 'no'. . . . One gets the feeling that the 'no' syndrome is active there," van der Merwe said in an interview this weekend.

His parallel to the Middle East referred to repeated Palestinian rejections of U.N.-sponsored resolutions that, theoretically, could lead to the eventual creation of a Palestinian homeland alongside Israel.

Van der Merwe contended that negativism could cloud the judgment of black leaders when they are offered opportunities for major changes in the political system.

His remarks came after virtually every black nationalist leader in the country, including moderates who already have been talking informally with the government, rejected out of hand a proposal for the election of blacks to serve on the National Council that would draft a new constitution giving the black majority a role in running South Africa.

Van der Merwe is an Afrikaner who supports reform and has taken the lead in the government of President Pieter W. Botha in trying to negotiate power sharing with blacks.

"Of course, some individuals want only to violently overthrow {the government} and grab power," he said. "I'm not talking about them, but a larger middle body that has goodwill potential. A layer of mutual suspicion covers this body of goodwill, and it makes it impossible to mobilize this goodwill."

Conceding that decades of discriminatory practices embodied in apartheid laws had led to this lack of trust, van der Merwe said he had no illusions about an early start for power-sharing negotiations, even once the National Council is created.

"The climate right at this time is not sufficient to get it off the ground in a satisfactory way," the deputy minister said. "But we need to get the machinery in place so it will be there at the right time."

Van der Merwe's views are considered important even by militant black nationalists because in the past nine months he has emerged as the most important government advocate of reforming the entrenched system of racial separation.

Senior officials of the outlawed African National Congress, in interviews at their exile headquarters in Lusaka, Zambia, have said they regard van der Merwe as a key factor in any negotiated settlement to the country's social and political crisis.

For his part, van der Merwe said he regards the ANC as a major player in any power-sharing talks, providing that it did not negotiate as an organization committed to the violent overthrow of the white minority government.

But van der Merwe, who is also Botha's chief information adviser, conceded that the ANC has not been willing to make such a concession. The ANC says it cannot negotiate while still legally outlawed, while political prisoners are being detained and while there is government violence against blacks.

Elaborating on Botha's public hint in August that imprisoned ANC leader Nelson Mandela might not have to renounce violence to be released, van der Merwe said renunciation of violence also would not be an "absolute" criterion for approving black candidates to the National Council.

"If the ANC would say, 'Yes, we are prepared to suspend violence' -- even though that might not be enough to enable the government to legalize them, it might be enough to enable us to start a process that could lead to that," the deputy minister said.

His statement appeared to reflect a further relaxation of the government's rigid preconditions for power-sharing talks with the ANC and other militant black groups. It also appeared to reflect the government's anxiety over the parliamentary elections in 1989.

Many political analysts believe that the ruling National Party, which has been in power since 1948, could suffer badly in the next election if it fails to make progress in finding a solution to South Africa's social and political crisis.

Despite the almost universal rejection of the National Council as a forum for discussing power-sharing, van der Merwe said he intends to try to maintain the momentum of gradual reform. Then, he said, what he sees as a moderate black majority will "recognize that there is a new fluidity in the situation."