El Salvador's President Jose Napoleon Duarte, dealing a potentially major blow to President Reagan's hopes for quick renewal of U.S. military aid to Nicaragua's contras, said yesterday that such aid should be withheld at least until January when an evaluation is to be made of the five-nation Central American peace agreement.

Duarte's unexpected statement to questioners at the National Press Club came as the contras' chief political leaders emerged from a meeting at the Capitol with Senate Minority Leader Robert J. Dole (R-Kan.) and announced they will send a delegation to Managua in the next few days in an effort to force Nicaragua's Marxist Sandinista government to deal with their U.S.-backed rebel movement.

Dole and several other Republican senators, who joined the contra leaders for their announcement, indicated that they would be accompanied by members of Congress when they attempt to fly to their homeland.

However, the contra leaders' statements created considerable confusion about whether their proposed move has the public support of the administration and of the four democratic governments involved with Nicaragua in the peace plan.

Guido Fernandez, the Costa Rican ambassador here, said last night that neither he nor Foreign Minister Rodrigo Madrigal Nieto had any information about the contras' plan and thus could not give an opinion on it. Fernandez added, though, that his government supports talks between the rival Nicaraguan factions, and he said that if the contras ask Madrigal to participate "in any mission to Managua acting as a mediator, he would accept provided it was acceptable to the Nicaraguan government and not a unilateral act."

The administration insists that the Sandinistas must deal with the contras about conditions for a cease-fire and political amnesty if the peace accord is to be implemented effectively by its Nov. 7 completion deadline. This U.S. position was given especially strong backing by Costa Rican President Oscar Arias in an interview published Wednesday by The New York Times. Arias' call for Sandinista-contra talks was given added moral authority by the fact that on Tuesday he was named the 1987 Nobel Peace Prize winner for his role as the chief architect of the plan.

Similarly, Duarte, here on a state visit, said in speeches yesterday to the Press Club and to an unofficial joint meeting of the House and Senate in the House chamber that the Sandinistas should follow his example in initiating a dialogue on amnesty and cease-fire with leftist insurgents fighting his government.

He said the accord, signed in Guatemala Aug. 7, calls for ending aid to insurgent forces until the agreement's effectiveness is evaluated by independent verificatiion teams in January. He added: "We have said, 'Give us a chance; give us this opportunity.' And the maximum time we ask from you is 150 days."

His timetable clashed with that of the administration which argues that quick renewal of contra aid is necessary to ensure that Nicaragua lives up to the agreement. On Tuesday, Secretary of State George P. Shultz said President Reagan will ask Congress for $270 million in new contra aid after Nov. 7 but before Thanksgiving.

However, congressional supporters of the peace agreement, led by House Speaker Jim Wright (D-Tex.), oppose contra aid while the peace process unfolds.

In revealing their plan, the contra leaders said they had asked for mediation by Cardinal Miguel Obando y Bravo, the Roman Catholic primate of Nicaragua. Contra leader Alfredo Cesar said, "We are starting today the hardball game in the peace process."

Nicaraguan President Daniel Ortega said in New York last week that his government would never negotiate with the contra leaders living outside the country. However, he noted that the Sandinistas have appointed a national reconciliation commission under the chairmanship of Obando, and he said any contras were free to return home and discuss questions of amnesty and cease-fires with representatives of the commission.

For that reason, some Latin American diplomatic sources said last night that both sides might be posturing to disguise that they are moving toward indirect talks conducted through the cardinal. But, the sources added, it may take a few days for the situation to clarify.

The contras did not say who would be in their delegation and when they would attempt to go to Nicaragua. However, Sen. John S. McCain (R-Ariz.) said he would accompany them. Sen. Richard G. Lugar (R-Ind.) said they were likely to go by the beginning of November, and he intimated that the administration might seek to dramatize the move by sending the aid request to Capitol Hill while the delegation is enroute.