WHILE President Reagan has been basking in Chinese fellowship, an interesting development has taken place in Central America. It may make things easier for Mr. Reagan there when he turns his attention back to that troubled area.
Jose Napoleon Duarte is the candidate of the moderate, centrist Christian Democratic Party. He seems to be doing well in the run-up to the decisive election due on Sunday. With some quiet background support from the United States Embassy the small fractional parties are either helping Mr. Duarte or promising their neutrality. He got 43 percent of the vote on the first-round election on March 25. He may well get over 50 percent in the finals.
If Mr. Duarte wins a clear majority and is allowed by the military to become president, he has promised to revive land reform, offer amnesty to the rebels, seek a dialogue with Nicaragua, prosecute the killers of the American Roman Catholic nuns, and put an end to the ''death squads.''
That is an ambitious program. Duarte could hardly expect, or be expected, to make spectacular headway quickly. But for him to set forth upon such a program would weaken the opposition in Congress to more aid for the government of El Salvador.
Congress is in no mood to underwrite more aid for a regime that so far has been unable or unwilling to identify and prosecute the murderers of the American nuns and many others. This is all the more true since last week when both a New York Times-CBS poll and a Gallup poll showed a majority of US public opinion opposed to US military involvement in Central America.
If the polls reflect reaction to the disclosure of CIA responsibility for blowing up oil tanks and mining harbors in Nicaragua, then the reaction has been negative. There is no evidence that the militarization of the American role in Central America has become popular enough to be an advantage to the President in the election campaign.
So Mr. Reagan would gain politically from a new regime in El Salvador seeking to end the civil war, reunite the country, and run a reputable, democratic system with regard for human rights.
The whole picture of events in Central America could be transformed by such a development. El Salvador would not have gone ''communist,'' but would have remained on the anticommunist side of the fence. Nicaragua would no longer be effectively exporting Marxist revolution to a neighbor.
A year ago President Reagan took a gloomy view of prospects in Central America. He said:
''If we don't stop the left in Salvador, we cannot expect to prevail elsewhere. Our credibility would collapse, our alliances would crumble, and the safety of our homeland would be put in jeopardy.''
Well, would failure in El Salvador prove to be that disastrous? Probably not. But so long as Mr. Reagan believes that the stakes are so high, he is bound to push forward with his military program. And the more he pushes ahead down that line, the more resistance he will encounter in Congress. The situation was getting tight when he laid all that aside and headed for his reconciliation with communism in China.
Mr. Duarte offers President Reagan a way out. Provided he wins on Sunday, provided the generals allow him to take office, and provided he is then allowed to make an impressive start down the road of this reform program - then a different prospect opens up.
Congress would support a reform president with a genuine reform program in El Salvador. Everyone (except presumably the Soviet communists) would applaud a turn in El Salvador toward reform and democracy. The President's differences with Congress over Central America would disappear, and Mr. Reagan could put aside his doomsday scenario. All it takes to dissolve that political nightmare would be a turn toward democracy and human rights in the key country that Mr. Reagan elevated to such importance.