When incurable cancer finally claims the life of President Jose Napoleon Duarte of El Salvador in a few months, the impact can be expected to be felt far beyond Central America.

In fact, the impact is already beginning to be felt now that Duarte has turned over his office to an acting president and come to the United States for treatment.This development means more than just the departure of perhaps the most pro-American president in Latin America. It also could mean the demise of key U.S. strategy along with much of the hope for the future of El Salvador.

Since Duarte's election four years ago, El Salvador has received $1.5 billion in U.S. economic aid and $455 million in military aid, levels unmatched elsewhere in Latin America.

So much aid was forthcoming because both the U.S. Congress and the White House saw Duarte as the best prospect for holding off a serious leftist insurgency in El Salvador while nurturing a legitimate civilian government to replace the military regimes that have controlled the country for 50 years.

Without Duarte, his Christian Democratic Party could become even more deeply divided than it already is. This split raises the prospect that victory in the 1989 presidential elections may go to the rightist ARENA party despite its reputed links to death squad operations.

Meanwhile, leftist insurgents remain a serious threat after nine years of sabotaging highways and electrical systems, impairing an inflation-riddled economy that stays viable only because of U.S. aid. The prospect is for more of the same chaos unless both sides can see their way clear to a cease-fire and peace negotiations.

In any event, the next President of the United States has some serious soul-searching to do about what direction Washington's policy toward El Salvador should take after Jose Napoleon Duarte.