Archbishop Pio Laghi, 58, a veteran of the Vatican diplomatic corps who is now serving in Argentina, will be the new apostolic delegate in this country.

The Italian-born prelate, who knows the American scene from his service as a junior staff member of the apostolic delegation here from 1954 to 1961, succeeds Archbishop Jean Jadot in the pivotal role as the eyes and ears of the pope in the United States. No date has been set for his arrival here, but church officials do not expect him before February.

For the church in this country, the choice of an apostolic delegate is crucial, since he is largely responsible for shaping the Vatican's impression of the U.S. church. More important, he is a key figure in the appointment of new bishops, who in turn determine the shape of the chruch here.

In the seven years he was apostolic delegate, Jadot oversaw the selection of 174 new bishops -- the vast majority of them men who had caught the vision of Pope John XXIII's Vatican Council and its spirit of reforms and renewal. The result has been a new mood of openness in the church in this country.

"I think we've got a winner," said the Most Rev. Thomas Kelly, executive secretary of the U.S. Catholic hierarchy. Kelly got to know Laghi during the conference of Latin American church leaders in Puebla, Mexico, nearly two years ago. The new man is "fast-paced," Kelly said. "He moves quickly, talks quickly and gets things done."

"We have had very good reports about his human rights record in Argentina," said the Rev. Brian Hehir, associate secretary for International Justice and Peace of the United States Conference of Catholic Bishops.

"The situation there is drastic," said Hehir, referring to reports of widespread human rights violations by the Argentine government. "Our information is that he has been very open to receiving the wives and mothers and family members of the 'disappeared,' as they call them. Of course he is the representative of the Vatican and there is a limit to what he can do."

Sister Gerald Hartney, who is currently coordinator of pastoral planning for the Washington Archdiocese, said she got to know the new delegate well while she was serving in Israel for her order, the Sisters of Holy Cross.

"It's the best news I've heard in a long time," she said. "He's one of the most delightful people I ever knew . . . He has a lot of polish, he has a warm heart, and he speaks English perfectly."

Before his assignment to Argentina in April 1974, Laghi served in Jerusalem, Palestine, India and Nicaragua.

Hartney said Laghi had excellent relationships with Israeli officials and with the Moslem community in Israel as well as with Christians. "Once I had a sister who had come from Rhodesia as a refugee on an international passport. The Israelis didn't recognize such papers and they were going to send he right back. He was able to intervene, and on his say-so, they gave her a laissez-passer, and she was able to stay," Hartney said.

During his tenure in the Holy Land, she said, Laghi "was instrumental in founding the University of Bethlehem for Moslems and Christian Arabs." Hartney added, "I'm Irish. The biggest compliment I can pay him is to say that if I didn't know he was Italian, I'd swear he was from Dublin."

Several people spoke of the new delegate's fascination with this country. "He's been hoping for this [appointment] for 25 years," said the Rev. Francis X. Murphy, rector of Holy Redeemer College and a keen observer of Vatican affairs. "He was always hanging around with the Americans when he was in Rome."

The Rev. Walter Schmitz, retired dean of the theology faculty of Catholic University, knew Laghi when he was here as an aide at the apostolic delegation. "He'll wear well; he meets people easily and he was well-liked when he was here," Schmitz said.

"I would classify him as a moderate," Schmitz said. "He will be very open and easy to deal with." Pointing to the new delegate's service in some of the hot spots of the globe, Schmitz observed: "It pleases me that it's not somebody who's been parked in Rome."

Laghi, Schmitz said, was the protege of then-Archbishop Amleto Cicognani, who was the delegate here from 1933 to 1958. "They came from the same home town in Italy," Schmitz said. "Cicognani got [Laghi] into school in Rome and into the diplomatic service . . . He came here as his first post."

Laghi was transferred from here to India where he served under Cardinal John Knox, Schmitz said. Knox, an Australian, has been in a high Vatican post since 1974, and in a position to help Laghi's career in the diplomatic service, Schmitz speculated.

The post of delegate to this country, which carries with it service as the Vatican's permanent observer to the Organization of American States, is very near the top of the tree in Vatican diplomatic careers. Traditionally, the next move is to the Vatican's governing Curia, with a cardinal's hat.

The previous delegate, Jadot, 70, was named in June as the pro-president of the Vatican Secretariat for Non-Christian Religions and is expected to be made a cardinal at the next consistory.

Church leaders concede that a sizable number of tasks have piled up since Jadot left the country in September. Among the major ones are the selection of a prelate to succeed Cardinal John Dearden of Detroit, who has retired, and resolution of the continuing problems in the Archdiocese of Chicago, the nation's largest see, where Cardinal John Cody has been at odds with a number of his priests.