ON the eve of South Africa's first full political negotiations, the major parties have agreed on a set of principles that will underpin the country's first nonracial constitution."I think you can say that exclusive white rule ends tomorrow [Friday] and several years of shared rule and political bargaining begins," says a Western diplomat. The first session of the so-called Convention for a Democratic South Africa (CODESA) - to be held Dec. 20 and 21 - represents the first national covention including all races. The 220 delegates from 19 political groups will determine the agenda for the talks, establish several working groups, and release a declaration of political intent. The talks will be boycotted by radical groups on both the political left and right. Despite the absence of the radical Pan Africanist Congress (PAC) and the right-wing Conservative Party, African National Congress (ANC) President Nelson Mandela was optimistic in a public speech on the eve of the talks. "The talks will succeed because all people of South Africa are behind us," he said. The agreed objective of CODESA is to negotiate a formula for transitional rule, draw up a code of constitutional principles, and determine the forum that will draw up the new constitution. The agreement on a set of common values and principles, known as a "declaration of intent," marks a historic compromise between the two main adversaries, the ruling National Party and the ANC. The National Party government will for the first time relinquish its monopoly on official decisionmaking and the ANC will recognize the constitutional authority of a Parliament that represents only the white, mixed-race, and Indian minorities. "This is a major concession on our part which has gone unnoticed so far," an ANC official says on condition of anonymity. The principles create a framework for constitutional talks that binds the parties to a multiparty system, regular elections based on proportional representation, an independent judiciary, and a fundamental bill of rights. They acknowledge the cultural diversity of the population and entrench the right of all to freedom of assembly, speech, and religion regardless of race, sex, or creed. The conference, representing at least 80 percent of the population, will make all key decisions on the basis of "sufficient consensus a concept of near-consensus which does not allow minority parties to obstruct a decision. "What this means in practice is that no major decision can be pushed through without the consent of at least the ruling National Party and the ANC," says an ANC official. "But racial rule will end only when there is an elected instrument to adopt a new nonracial constitution." ANC officials say the process will take no longer than 18 months. Government officials say it could take longer. The third-largest grouping in the talks, the Zulu-based Inkatha Freedom Party, is closer to the government than is the ANC. IFP President Chief Mangosuthu Buthelezi said in an interview Dec. 16 in the British tabloid Daily Star that an ANC government would lead to bloodshed in South Africa. "The whites in this country will not stand for it," he said. "They are armed, they are mobile, and because of conscription, they are all trained soldiers." A demand by Chief Buthelezi that Zulu King Goodwill Zwelithini be allowed to lead a separate delegation to the talks appears to have been defused by a proposal by Mr. Mandela that all traditional leaders be allowed to attend as observers. The right-wing Conservative Party, which represents some 30 percent of white voters, has vowed to boycott the talks until CODESA acknowledges the white right to "self-determination." The CODESA principles, which will be formally adopted at the meeting, commit all parties to abide by the decisions and to promote their implementation. But the government has balked at Mandela's insistence that the decisions should be binding in law. "CODESA's decisions in themselves cannot be legally binding," said Constitutional and Development Minister Gerrit Viljoen on the eve of the talks. "The only way that that can happen is through the existing constitutional structures of Parliament," the minister added. But he added that all CODESA participants would be bound by agreement to promote the implementation of decisions. Despite the repeal of most apartheid laws, questions over the role of security forces in ongoing political violence have delayed the opening of talks and created mistrust between government and the ANC. The talks will be attended by high-level delegations from the 166-member United Nations, the 50-member Commonwealth, the Organization of African Unity, and the Nonaligned Movement.