At a time of growing political and economic turmoil in Nicaragua, the ruling Sandinista Front has sent its top foreign-policy adviser to Washington to seek better relations with the Reagan administration and try to quiet doubts about the course of the revolution among its onetime allies in Congress and elsewhere.

Julio Lopez, international relations secretary of the Sandinista National Liberation Front, said the principal purpose of his visit was to push Nicaragua's proposals for a negotiated settlement of the conflict in El Salvador. But he spent much of his time both at the State Department and on Capitol Hill defending the Nicaraguan government's crackdown on political opponents.

A major topic in many of his meetings was the arrest Wednesday of four leading businessmen who were well-known on Capitol Hill, where they had lobbied for aid to Nicaragua since the Sandinistas took power more than two years ago. One of the four, Enrique Dreyfus, chairman of a powerful business federation, had worked actively for a $33 million aid package passed by the Senate the day before he was arrested. An amendment by Sen. Edward Zorinsky (D-Neb.) provided that all aid be channeled to the private sector until the Nicaraguan government gives assurances that it will hold elections.

Lopez said Dreyfus and the other three businessmen were arrested for violating a law declaring a state of economic emergency and for making statements that would weaken the country's shaky economy. The businessmen issued an open letter to junta leader Daniel Ortega Monday saying the government was following "an unmistakable Marxist-Leninist ideological line" and statements by some government leaders "can only be interpreted as the preparation of a new genocide in Nicaragua."

Lopez said in an interview that in their trial, now under way, the businessmen will be required to prove these statements.

Participants in Lopez's meetings with congressmen said he was also asked to explain the recent 48-hour closings of the opposition newspaper La Prensa and a private radio station, as well as the seizure of the passports of three opposition leaders who had planned to tour Western Europe seeking political support.

Lopez said in the interview that Nicaragua had shown "extraordinary maturity" in dealing with political opposition in the face of severe economic and political tension:

"Do we kill and torture our opponents as is commonly done in Central America? We put them on trial under legal procedures. We have many newspapers, private radio stations, and private enterprise is the dominant force in the economy."

Rep. Millicent Fenwick (R-N.J.), who attended a lunch for Lopez Tuesday, said, "It is awfully difficult for the United States to contemplate developments in Nicaragua with much hope or confidence. An emergency law that permits the jailing of people in this way is very discouraging. I voted for Nicaraguan aid before, but now I wouldn't vote to give them a dime."

Rep. Michael Barnes (D-Md.), chairman of the House inter-American affairs subcommittee, who met with Lopez yesterday, said the dropping away of support for the Sandinistas on Capitol Hill reflected a similar disillusionment in other countries.

The presidents of Costa Rica and Panama, two countries that have supported the Sandinistas since before they ousted dictator Anastasio Somoza, have called for the release of the four businessmen. Venezuela, which has provided substantial aid to Managua, has urged the Sandinistas to hold elections.

Nicaragua's relations with the United States, which seemed to improve briefly after a visit to Managua Aug. 12 and 13 by Assistant Secretary of State Thomas Enders, have cooled again in recent weeks.

State Department sources have charged that Nicaragua is permitting large-scale arms traffic through its territory to the leftist insurgents in El Salvador and that several hundred Cuban troops have recently arrived in Nicaragua.

Lopez said the report of Cuban troops is "a ridiculous joke. There are no Cuban troops in Nicaragua." He said the government was trying to stop arms traffic to El Salvador.