MANAGUA, NICARAGUA, SEPT. 17 -- Secretary of Education William J. Bennett marked the 200th anniversary of the U.S. Constitution by visiting Nicaragua and reaffirming the administration's commitment to rebels, known as contras, fighting to overthrow the government.

"We will support the contras," Bennett said at a midday press conference. "To abandon the contras is to enter on an irreversible course. Once abandoned, they are lost."

The trip by Bennett, who is also an authority on American political history, was described as a presidential mission. He was the only Cabinet member to travel overseas today on behalf of the White House to mark the anniversary of the Constitution, said his press secretary, Loye Miller.

Bennett's day-long stay coincided with the opening of two meetings here to discuss implementation of a peace plan signed Aug. 7 in Guatemala by the five Central American presidents. The five regional foreign ministers met, as did a group of Latin American deputy ministers and representatives from the United Nations and the Organization of American States, members of a commission that will oversee compliance with the accord.

While the foreign ministers expressed optimism about the accord's progress, Bennett amplified the administration's objections to it.

The five presidents "are going by their lights, but we must go by ours. We believe we must honor our commitment to the contras because they are an essential pressure point for reform," Bennett said.

The Guatemala pact calls, among other points, for an end to outside aid to rebel armies and for full democratic freedoms in each country. Bennett said it was a mistake in the plan that is does not call for an end to the Soviet Bloc presence in Nicaragua.

The administration is seeking $270 million in new aid for the contras. Congressional Democratic leaders have accused the administration of ignoring Central American peace efforts by pushing for the aid before the Nov. 7 deadline set for compliance with the accord.

Taking a jab at Democrats who oppose contra aid, Bennett reported that two Nicaraguan human rights leaders told him: "The Nicaraguan people think ideas here are conceived in Moscow, directed in Havana, implemented in Managua and defended in Washington by many Democrats in the U.S. Congress."

The education secretary, who switched from Democrat to Republican in 1986 in response to Democratic opposition to contra aid, did not meet any government officials. He talked with opposition press and human rights leaders and with Cardinal Miguel Obando y Bravo, the most influential critic of the Sandinista government. He made a speech about the Constitution to a group of Nicaraguans.

Bennett said there was "nothing overly coy or subtle" about the White House choice to send him to Nicaragua, where he said the freedoms protected by the U.S. Constitution "do not obtain."

About 50 Sandinista sympathizers and disabled war victims, many in wheelchairs, set up a street protest outside a building where Bennett was scheduled to meet with relatives of political prisoners. U.S. Embassy officials accompanying Bennett decided he should not try to press pass the demonstrators. The embassy will file a formal protest with the government Friday, Miller said.

Bennett's was the latest of several visits here since Aug. 7 by U.S. Republicans who faced no obstacles from the leftist government in coming to Nicaragua and criticizing its Sandinista rulers. Senate Minority Leader Robert J. Dole of Kansas had a spat with President Daniel Ortega in front of reporters in Managua Aug. 31.