THE publication of the ruling National Party's proposals for a democratic constitution has opened a season of intense bargaining before negotiations begin over a transition to majority rule."After two years of hard work we are on the threshold of negotiation," President Frederik de Klerk told delegates at the party's annual conference in Natal province on Friday. Mr. De Klerk said he was confident it was possible to have a new constitution within three years. If there is no agreement by then, South Africa faces a national ballot that would risk a victory by right-wing parties. The proposals were approved at the ruling party's national conference here Thursday but must still win the support of the four provincial party congresses before they become policy. The National Party plan, has been sharply criticized by the African National Congress and other groups as "apartheid in disguise," a charge De Klerk denies. He said his party would negotiate in good faith but would use its power to ensure a system does not emerge in which "one group would dominate another." "The proposals are a recipe for continued conflict," ANC President Nelson Mandela said Friday. "They are proposals that, if advanced seriously, can only retard progress toward a negotiated solution." The ANC's proposals advocate a centralized system in which protection for minorities would not frustrate majority rule. The most controversial features of the National Party plan include a minority veto in the upper house, proportional representation in the Cabinet, and a rotating presidency in an executive council of three to five members. National Party officials justify these mechanisms as a bid to force rule by a "grand coalition" of the major parties. Also under fire are proposed neighborhood councils - with powers to determine "norms and standards and a system of voting at municipal level that gives extra votes to p roperty owners and taxpayers, who are predominantly white. Mr. Mandela said the National Party plan would "entrench minority privileges by ensuring that any majority party is powerless to make significant social changes." Prominent white liberals have also been critical of the National Party's proposals. "In their present form they would prevent a fundamental transformation of the society," says Democratic Party legislator Jan van Eck. "It is only when such a transformation is guaranteed that you can begin to talk about checks-and-balances with any legitimacy," he says. But political scientists say there is much in common between the two sets of proposals. "There is already a fairly impressive degree of convergence on basic democratic principles," says David Welsh, political scientist at Cape Town University. "The central point is that the National Party has accepted universal suffrage on a common voters roll in the lower house of Parliament. Once that linchpin is in place, bargaining about the final form of the constitution is made much easier." The more controversial proposals would not survive the negotiating process, Professor Welsh says. "Proposals like the minority veto in the upper house will not fly in their present form." The liberal Democratic Party's own proposals are closer to the ANC plan but have much in common with both. "When you boil it down the Democratic proposals represent the point of compromise between the National Party and the ANC," says a Western diplomat. All three plans propose a multiparty democracy with an independent judiciary and a bill of human rights. They all opt for a two-house parliament based on one person, one vote elections in the lower house and regional representation in the upper house. They all advocate the supremacy of the constitution as the ultimate authority in judging both the legislative and the executive. The Democratic Party proposes a federal constitution and spells out how the powers of government will be divided between federal and state governments. The National Party leans toward a federal system, while the ANC leans toward retaining more power in central government while still providing for "strong and effective local government." The degree to which power is centralized is likely to become the main dynamic of the negotiations. The next stage of the negotiating process will be a national peace conference next weekend in Johannesburg which will lay the foundation for an all-party conference to discuss constitutional principles, the form of transitional rule, and what body will draw up a new constitution.