President Jose Napoleon Duarte of El Salvador said yesterday that his longtime archrival, rightist leader Roberto D'Aubuisson, "will fade away" soon because times have changed in El Salvador.

The former army major, who lost to Duarte in the 1984 presidential election, "arose at an historical juncture and played a role at a certain time, and his time has passed," Duarte said. "In my opinion, he will fade away."

Speaking at a lunch with Washington Post editors and reporters on the second day of a nine-day U.S. visit, Duarte said he is confident that he can push judicial-system reform and a revised criminal code through the legislature, which El Salvador's rightists controlled until elections last March.

D'Aubuisson remains a legislature member, but his ARENA party is badly divided, and Duarte's Christian Democrats have a majority.

However, the nation's supreme court "is in the hands of the extreme right completely," Duarte said, and it recently freed a key figure in the 1980 murders of two U.S. labor advisers and their Salvadoran counterpart. Similarly, the nation's court system is corrupt," he said.

Duarte spoke as U.S. labor leaders who recently returned from El Salvador told the House Foreign Affairs subcommittee on international operations that conditions for trade unionists there are so bad that no further U.S. military aid should be sent.

A Salvadoran attorney "may have been telling the truth" when he said recently that he was threatened with death if he pursued his investigation into the 1980 murders of four U.S. churchwomen, Duarte said, because threats are common in Salvadoran proceedings.

But, he added, "I am absolutely convinced" that no higher officials were involved in that case. "There is no cover-up," Duarte said.

A new judicial commission is to revise the law, and another is to organize an investigating unit similar to the Federal Bureau of Investigation, Duarte said.

Duarte insisted that he has not come to the United States to request more military or economic aid "at this time" but rather "to tell Americans how their aid has been used." The United States has given El Salvador $1.5 billion in aid since 1981 and has proposed $483 million this year.

Duarte said he would ask Agriculture Department officials to rescind a recent reduction in El Salvador's U.S. sugar-sale quota and would try to renegotiate $27 million in interest payments coming due on recent military loans.

A State Department official said that the sugar quota was recently cut slightly for every country in the Caribbean region and that El Salvador is not being singled out.