TWO OF THE most sinister figures in Haiti's recent history have now returned there from exile. Their presence hints at an attempt to reestablish something along the lines of the bloody-handed Duvalier regime, which ruled by violence and terror for three decades until a popular uprising overthrew it four years ago. Ever since then the country has been caught in a deadlock between the democrats, pushing for elections, and the old Duvalierists fighting to preserve the privileges and graft on which they have been living for a generation.

Of the two returned exiles, the more dangerous is Roger Lafontant. He was interior minister under Jean-Claude Duvalier and reportedly the head of the Ton-tons Macoutes, who were the regime's gunmen and enforcers. The other, Gen. William Regala, held the same office under the military government when gangs of armed men shot up the polls in the presidential election of 1987, killing dozens of voters while soldiers and police looked on. Haiti has yet to hold a legitimate election for president.

But it is now going to try again. Elections are scheduled for November. The present provisional government is led by a civilian, Ertha Pascal Trouillot, whose chief responsibility is to see that those elections are actually held. But after the 1987 experience, that won't be likely unless the government can assure voters of security at the polls. And it is going to have trouble doing that with people like, especially, Mr. Lafontant walking freely through the streets. There's a warrant out for his arrest, and he is in effect daring the government to attempt to enforce it.

Haiti's long tradition of predatory government has left it by far the most wretchedly poor country in the Western Hemisphere. Driven by desperation, people there are taking the same risks to flee as the Vietnamese boat people. One day last week 39 Haitians were drowned in an attempt to get out, when a Bahamian patrol vessel intercepted them and their boat capsized. Haiti's misery is becoming a threat to the stability of the island democracies around it.

At a time when American politicians are loudly celebrating the revival of democracy in Eastern Europe, they are doing very little for this direly threatened country off the coast of Florida. Not only Haiti's neighbors but also the United Nations have responsibilities here. The U.N. can, if it chooses, make an important contribution to fairness and security in these elections. They are the great opportunity for progress in Haiti, and if they fail it may be a very long wait for another one.