THE United States ratcheted up the trade embargo on Haiti this week in what is becoming a test case of hemispheric enforcement of democracy.It will be a difficult test case. The embargo is aimed at reinstating the Caribbean nation's first democratically elected president, Jean-Bertrand Aristide, who was ousted in a military coup Sept. 30. It is the first exercise of a new Organization of American States (OAS) mechanism, modifying the group's longstanding principle of nonintervention, that permits it to confront violent overthrow of democratic governments in the region. The embargo, banning all commercial trade except basic food and airline service, is expected to bring the Western Hemisphere's poorest nation to a painful economic halt in short order. But diplomats at the OAS, where the international effort is centered, are struggling to gauge how and at whom to aim the negotiating efforts accompanying the embargo. "One of the actually difficult aspects of Haiti is that a lot of different people will give a different answer to the question of who exactly is in charge here. There are situations where sometimes no one is fully in charge," says Luigi Einaudi, US Ambassador to the OAS. Indeed, the roil of Haitian politics with its blend of flash-fire violence, shifting loyalties, raw greed, and even earnest, if naive, visions of democracy makes negotiation a fragile and dangerous process. Recent Haitian history is littered with the names of those who held power and the agreements they made. Since dictator Jean-Claude "Baby Doc" Duvalier was ousted in February 1986, seven governments - five the result of coups - have laid claim to "democratic" ambitions. Only President Aristide was democratically elected, and even he has come under international criticism for condoning - even inciting - mob justice. So international efforts to support the nation's democratic experiment are frequently frustrated by the dynamics of the Haitian political scene - and the current situation before OAS negotiators is no different, observers say. There have been two OAS missions to Haiti since Sept. 30. A third OAS civilian observer mission, headed by former Colombian Foreign Minister Augusto Ramirez Ocampo, has been invited by the Haitian Senate to go there within the next week. The embargo is calculated to break the resolve of the military and many members of the Haitian civilian elite who have vowed not to allow Aristide to be reseated. But what then? Scenarios bandied about by observers of Haiti include negotiated settlements that involve Aristide returning to power only so that he can leave it in a constitutional way; restoring him to power with a long-term international civilian observer group or an armed international force to keep the peace while a new police institution is built. Ambassador Einaudi says that the shaping of the civilian observer mission to Haiti has been the subject of initial OAS talks with Haitian officials. He underscores the fact that the OAS group is a civilian group, but adds that it will have a security element for the safety of the mission itself. It will "require more than a handful of people to have any kind of effective monitoring and institution supporting effect." Aristide, who as president and as a radical Roman Catholic priest was hugely popular with the poor masses, survived numerous assassination attempts, and would most certainly be in danger if he were to return, virtually all observers agree. "Aristide can be restored only with an international peacekeeping force. How else can he return with a very hostile army and political elite?" asks Raymond Joseph, copublisher and editor of the New York-based Haiti Observateur and former charge d'affaires in Haiti's Washington embassy. The chaos that the trade ban will generate combined with the passion of Aristide's followers is likely to cause antimilitary and anti-elite violence too, he adds. Mr. Joseph's call for a peace-keeping force echoes a call increasingly heard over the Haitian problem. The presidents of Argentina, Venezuela, and the prime minister of Jamaica have all called for a multilateral peace-keeping force. Ultimately the OAS is trying to jump-start a democratic engine that had a little fuel - a president enthusiastically elected by 67 percent of the voters - but not all the parts, like a democratic tradition and institutions, to make it run. The whole notion of compromise over Aristide's return may even be impossible, observes US Rep. Jim Oberstar (D) of Minnesota, a member of the Congressional Task Force on Haiti who speaks Haitian creole and taught English to military officers in Port-au-Prince in the 1960s. "We are ready to compromise on just about everything ... even on positions of principle. Haitians, with no democratic tradition and legislative experience are not in the same mold," says Mr. Oberstar, who supports a peacekeeping force in Haiti. "Haitians may say democracy, and they mean elections. We say democracy, and we understand a whole process with the baggage of 200 years [experience]." Many observers see the Haitian problem as a turning point for hemispheric security. The international response is a sort of warning to the democracies in the region with wavering militaries, observers say.