Senior Nicaraguan officials asked House Speaker Jim Wright (D-Tex.) Wednesday to mediate treaty negotiations between the Sandinista government and U.S.-backed contra rebels, Washington sources said yesterday.

Wright declined, saying that he does not have the time and that he felt any intermediary should come from Central America.

But sources familiar with the attempt to arrange a cease-fire said they interpreted the request "as confirmation of the true seriousness of the Nicaraguan government about the cease-fire."

On Friday, Nicaraguan President Daniel Ortega asked Roman Catholic Cardinal Miguel Obando y Bravo, the archbishop of Managua and an outspoken critic of the Sandinista government, to serve as intermediary.

The contra leaders have said that the cardinal is the only intermediary they will accept in the talks with the Sandinista government.

The talks grow out of a five-nation Central American peace plan signed on Aug. 7.

The cardinal has not yet decided whether to accept the government offer, saying that "certain steps must be taken to see how this will be done."

Sources said that Nicaraguan deputy foreign minister Victor Hugo Tinoco flew to Washington to seek Wright's intervention.

In their meeting he said it was essential that the cease-fire be carried out and he promised the speaker that the Sandinista leaders intended to comply.

Tinoco told Wright that they went to him because the Sandinistas were convinced that somebody of international stature was needed to ensure the success of the negotiations.

The request by Ortega to the cardinal on Friday came as a surprise, because Ortega had said he would not hold indirect cease-fire talks with the contras. Until yesterday he had insisted on meeting only with representatives of the U.S. government.

Reagan administration officials have responded to Ortega's interest in negotiations with suspicion.

An administration official who declined to be named said yesterday that "seeking to involve the speaker directly in the negotiations is the type of act that carries few political risks. Clearly, he could not accept and the Sandinistas walk away looking sincere."

But Senate Democrats monitoring the implementation of the new Central American agreement said Friday that seeking an intermediary was a significant break in the negotiations.