NINETEEN months of arduous United Nations-sponsored talks in Geneva, Caracas, San Jose, and Mexico City have not ended two decades of war in El Salvador. On Monday the venue of the negotiations moves to New York City.Many people hoped for a definitive peace treaty last April, when an initiative on proposed constitutional changes offered by the Farabundo Marti National Liberation Front (FMLN) - an umbrella group of five leftist groups - was seriously, but unsuccessfully, discussed. Five months later, UN Secretary-General Javier Perez de Cuellar, a Peruvian, appears concerned that his successor next year - probably not a Latin American - will have priorities other than untangling this Gordian knot. In August, Perez de Cuellar wrote the United States and Soviet foreign ministers for assistance and subsequently accepted their request to become personally involved in the talks Sept. 16-17 in New York. If the September talks do not resolve the increasingly unstable situation in Salvador, elements on both sides want to intensify the fighting, particularly while their armaments remain well stocked. The government policy could also shift with what appears to be the imminent death of Roberto D'Aubuisson, founder of the ruling ARENA party. Six party leaders are vying to be the new party "strong man" by espousing hard-line positions. Mr. D'Aubuisson, who has been frequently accused of organizing death squads in the early 1980s, is credited with making most of ARENA's important party decisions. The ARENA government policy since Alfredo Cristiani was elected president in 1989 has been unexpectedly moderate, given D'Aubuisson's reputation. The UN talks have been marred by misunderstandings, posturing, and shifting demands by both sides as internal factions contend over rapprochement. Military offensives and war-related atrocities have further inflamed emotions. Yet virtually no successful attempt at national reconciliation has been violence-free. A "leap of faith," forgetting past antagonisms, is as necessary as it is risky. The 1988 return of Ruben Zamora and Guillermo Ungo from exile showed that reintegration into political life can be possible for FMLN leaders. The participation of their respective parties in the 1989 presidential and 1991 legislative elections - despite charges and counter-charges of election fraud and irregular registration practices - bolstered the democratic left. Ten legislators on the left were elected last March. The 90 deputies of the National Assembly, where no single party has a majority, have demonstrated that vast differences on economic issues need not preclude respect for debate and compromise. UNFORTUNATELY, the virtually nonfunctioning criminal-justice system is a reminder of the de facto immunity for continuing repression, no matter how much progress the legislature has achieved. Mr. Zamora accurately claims that many in the government do not yet accept the rights of left-oriented parties, unions, and cooperatives to undertake nonviolent political action. The Army responds, with some truth, that some of those organizations are supporting the guerrillas militarily, as the November 1989 FMLN offensive in the capital amply demonstrated. What is needed is an exit from such endless excuses for war. One hopeful moment came at a recent unofficial conference in Costa Rica. Early in the meetings, prominent Salvadoran politicians, businessmen, and union leaders expressed an apparent consensus in favor of mutual demobilization down to reasonable peacetime levels. Regrettably, as soon as Salvadoran Defense Minister Rene Emilio Ponce and FMLN negotiator Jorge Shafik Handal arrived, outspoken support for demilitarization ceased. John McAward, who has visited the region for two decades, observed: "Their unprecedented, private meeting of 45 minutes can be considered progress, but neither Ponce nor Handal sounded much like democrats. They appear more interested in preserving or enhancing their own power than respecting the constitutional powers. Still, obtaining a commitment to democracy could change past practices." To avoid the "catch-22" of endless war and repression, both negotiators understand that they must accept the 1994 elections, when seats will be contested at all levels of government. Refinements in electoral, demobilization, and constitutional processes are needed but need not necessarily be forged prior to a cease-fire accord emerging from New York. Distrust among all the parties works to prevent a desperately needed truce. Disagreements over the timing and extent of disarmament and amnesties, for example, may have to be assigned to the legislature, while relying on the UN to provide ample warning if either side undertakes systematic military action. Seemingly irresolvable situations involving elections in Haiti and Nicaragua were vastly improved through UN monitoring over the past two years. Because of the greater intensity and duration of fighting and human rights violations, and more ambiguous past electoral mandates, El Salvador presents a more complex challenge. Initiating peace requires the guerrillas to accept a schedule for disarmament in a highly imperfect, though improving democracy. The Salvadoran military must abandon its attempt to exterminate the rebels. The US, in particular, should increase its pressure on the military to implement major reductions, purges, and reorganizations. Otherwise, the unavoidably long-term work of mitigating injustice cannot begin.