JOHANNESBURG — TWO years after President Frederik de Klerk reached an understanding with then-jailed African National Congress leader Nelson Mandela, the country's political leaders have sealed an accord leading to the start of full political negotiations on Dec. 20.The accord was underwritten by 19 out of 20 political groups that attended a two-day meeting marked by a spirit of compromise and reconciliation. The Convention for a Democratic South Africa (CODESA), as the forum is to be known, will determine a set of constitutional principles, negotiate a transitional authority, and decide on the eventual forum that will draw up the new Constitution. The weekend meeting set up a 20-member steering committee that will convene the Dec. 20 meeting, to be chaired by two judges, Ismail Mahomed, South Africa's first black judge, and Chief Justice Petrus Schabort. With 240 delegates from 20 political groups, it will be the most representative political forum in the country's history. It marks the beginning of a period of democratization after two years of liberalization that has seen the removal of apartheid and a relaxation of repressive laws. The government, which was represented by a separate delegation from the ruling National Party, had to endorse several compromises concerning the venue, conveners, and chairmen of December's meeting. The right-wing Conservative Party (CP) and the left-wing Azanian People's Organization opted to stay away. The Pan Africanist Congress (PAC), radical rival of the African National Congress (ANC), walked out just before the meeting ended pending a review of strategy at its Dec. 16 national conference. But the accord was endorsed by the three major players - the National Party, the ANC, and the Zulu-based Inkatha Freedom Party. ANC Secretary-General Cyril Ramaphosa, who headed the ANC team during the talks, said the historic accord marked "the opening of genuine negotiations. We are convinced that we are walking the last mile," he said, noting that delegates had risen above party political interests. "This commitment to the national interest that has inspired us over the past two days is the only assurance that we have that we shall achieve democracy." Constitutional Development Minister Gerrit Viljoen, who headed the government team, said the accord marked a clear break with the past. "This is a watershed in the history of our country. The chemistry of mutual trust and confidence has begun to work and it is most encouraging." The accord came two days after a parliamentary by-election in rural Orange Free State province, where the Conservative Party captured a marginal National Party seat with a 3,000-vote majority. The Conservative Party demands that Mr. De Klerk call a general election that it claims it would win. Under the present Constitution, De Klerk has three years before he must hold an election. He has vowed to negotiate a democratic Constitution before 1995 that would drastically reduce the influence of the white vote by extending the franchise to the black majority. But he has said he will implement the new Constitution only if he wins a majority among whites in a national referendum that will include all races. The Conservative Party is deeply divided over strategy toward negotiations. Koos van der Merwe, a CP legislator who favors participation, made a surprise appearance at the hotel where the talks were held. He told reporters that the CP would join the talks if the conference acknowledged the white right to self-determination. The PAC, which recently declared itself in favor of negotiations, complained the conference was rigged following what it claims were secret deals between the ANC and the government. The government and the ANC denied the charges, but confirmed extensive bilateral talks were necessary to ensure the smooth running of the meeting. Officials from both parties predicted the PAC's withdrawal would be temporary and that they would return to the negotiating table for CODESA's inaugural meeting. "We are entitled to review our position," PAC Deputy President Dikgang Moseneke said, adding that the PAC had no objections in principle to negotiations. "Where critical national issues are involved, we will remain engaged in the process."