Nicaraguan President Daniel Ortega invited U.S. congressional leaders last night to form a bipartisan commission that should visit Nicaragua and find out that his country's military development is purely defensive.

Ortega extended the invitation as Congress nears a vote on the renewal of U.S. aid to Nicaraguan rebels who have been seeking to oust the Sandinista government in Managua.

The Nicaraguan leader said that Nicaragua was seeking to renew talks with the United States and the regional talks by the so-called Contadora group.

Ortega, who will attend the inauguration of Julio Sanguinetti as president of Uruguay on March 1, also said he will announce in the Uruguayan capital of Montevideo new Nicaraguan proposals that will form part of the Contadora peace negotiations.

The announcements came after Ortega held two meetings with five U.S. bishops who represent the National Conference of Catholic Bishops. He did not give details on the new Contadora proposals, but Bishop Rene Gracida of Corpus Christi, Tex., called the Nicaraguan proposals "substantive concessions."

"The bishops were impressed, as a group, with the extent and the boldness of the initiative," said Gracida, but he also refused to give details.

The Contadora peace process, named after the Paraguayan island, where the first meeting was held, involves Colombia, Mexico, Panama and Venezuela.

Ortega said that letters will be sent today to Senate Majority Leader Robert Dole (R-Kan.) and House Speaker Thomas P. (Tip) O'Neill Jr., (D-Mass.) proposing to form a bipartisan commission to study the Nicaraguan military situation.

"We are inviting them to form a bipartisan committee that will come to Nicaragua without any type of restrictions, going wherever they want, so they can know the reality of the military development that exists in Nicaragua, which is a truly defensive development," said Ortega.

Ortega extended the invitation apparently in response to repeated charges by the Reagan administration that Nicaragua represents a danger to its Central American neighbors and possibly to the United States.

Congress is expected to vote in late March or early April on $14 million in aid to the counterrevolutionaries, known as contras. Congress cut off official aid to the rebels last July, but President Reagan is campaigning for approval of the $14 million. The Sandinista government is lobbying Congress to continue the ban on aid to the contras.

The five American bishops, who are on a fact-finding tour here, have reiterated that they have never supported the granting of military aid "to any faction involved in any conflict anywhere."

The clergymen also have voiced their support for peace talks involving all opposition forces, an option which the Sandinista government has repeatedly rejected. The Sandinista leaders say they will not negotiate with the guerrillas, whom they describe as mercenaries armed and directed by the United States.

Bishop Gracida told reporters that during an earlier meeting with independent human rights officials, who enumerated alleged Sandinista abuses, it was the U.S. bishops who asked for details about abuses allegedly committed by the contras. He said in that meeting, and in another with Sandinista leaders, the bishops heard descriptions of violations of human rights by both sides.

Archbishop James Hickey of Washington told journalists that he believes a preliminary report on the bishops' tour should be out before the expected congressional vote.

The bishops have repeatedly voiced their opposition to the militarization of the conflicts in Central America. Besides Gracida and Hickey, the delegation includes Cardinal Joseph Bernardin of Chicago; Archbishop John O'Connor of New York, chairman of the bishops' Committee on Social Development and World Peace, and Bishop Sean O'Malley of the Virgin Islands.

The bishops told journalists that they had asked Ortega to try to resolve certain problems related to them by the Nicaraguan bishops.

Relations between the government and the Catholic Church hierarchy have been tense since soon after the Sandinistas came to power in 1979. Last July, the Sandinistas deported 10 foreign priests working in Nicaragua, who were accused of preaching opposition to the government.

O'Connor, spokesman for the U.S. bishops, did not mention the expelled priests but did say that the Nicaraguan bishops wanted the right to visit prisoners in jail and also to say mass on radio and television.