THE chances that Jose Napoleon Duarte can lead El Salvador into a peaceful and modern future have always been slim, and they will remain slim and uncertain after his visit in Washington this week.
But those chances are less slim, indeed almost promising, because of the sequence of recent events concerning his country. The key fact is that a reluctant United States House of Representatives voted money for President Reagan's proposed aid to El Salvador after and also because of Mr. Duarte's election.
Had he been defeated by his rival, Roberto d'Aubuisson, the House would have refused to appropriate the money. In other words, by getting himself elected, Mr. Duarte became the man from El Salvador who could get the money from Washington. His rivals could not have done it.
Hence, in the eyes of the right-wing generals and political leaders Mr. Duarte has become important and useful. They need him. They are willing, at least for the time being, to tolerate him, because otherwise the funds would not be available for continuing the battle.
Mr. Duarte is now in the promising position of being not only the manifest preference of a majority of the people of El Salvador as their President, he is also in the position of paymaster. He got the money. He has potential control of its spending - for the moment.
For him to go on from here and lead his country into a happier and more peaceful future, it is essential that he continue to be in the position of both paymaster and armorer. So long as the generals must depend on him for the money to pay the troops and for the weapons to arm them, President Duarte will be a President with power.
But if at any time Washington forgets the logic of the present situation in El Salvador and lets funds and weapons go directly to the generals, Mr. Duarte will become just a puppet, and the reforms he proposes will languish. If they languish and the ''death squads'' continue their grisly work, the willingness of Congress to support El Salvador will be washed away.
In effect, there is a deal between the Democrats, who control the House in Washington, and the Reagan Republicans, who control the White House and the Senate. The Democrats will support the rescue operation in El Salvador as long as it is led by Mr. Duarte. Therefore it is now up to the White House to see to it that Mr. Duarte has the means to win at least the tolerance of the more right-wing political elements and also the actual obedience of the generals.
This is Mr. Duarte's second election to the presidency. His first election was spoiled when he tried to push reforms too far and too fast to suit the far right. They pushed him aside. It is to be both presumed, and hoped, that he learned a lesson from his first failure. He is likely to go more slowly this time.
Also, he was not given the support the first time which President Reagan is of course promising him this week in Washington. He did not then have the leverage he has now as the paymaster of the troops.
As in other Central American countries, the long-term problem in El Salvador is the relationship between resources and space. The population of El Salvador has 10 times the density of the US population and one-tenth the per capita annual income. El Salvador is not the poorest country in Central America. Honduras is worse off, with an annual per capita income at $549. The comparable figure for El Salvador is $760 and for the US it is $9,637.
Mr. Duarte is not going to be able to raise the average standard of living quickly or greatly. To do that would require both drastic population control and an enormous capital investment. No one in Washington is proposing or contemplating the type of program for Central America that could bring it into a state of affluence.
But if Mr. Duarte can get the backing of the conservative forces, he ought to be able to reduce the violence and make headway against the rebellion. Even small progress toward peace, law, and order would be a long step forward and could win him general support. But for him to get ahead, Washington must channel its aid through him.