Pope John Paul II put his stamp of approval on an aggressively conservative church leadership yesterday with his choice of 28 new cardinals.

The two Americans, Archbishops John J. O'Connor of New York and Bernard Law of Boston, distinguished themselves in Vatican eyes during last year's presidential election by challenging Catholic voters to make the church's opposition to abortion the criterion for selecting political candidates.

When O'Connor and Law were named to their respective posts a little more than a year ago -- each from modest previous assignments -- it was a foregone conclusion that each would one day re-ceive the red hat, since new cardinals are customarily named in dioceses previously headed by a cardinal.

Both New York and Boston are in that category. But so are Washington, St. Louis, Baltimore and Detroit. The relatively progressive archbishops of each of these archdioceses have now been passed over twice in the naming of new cardinals.

Washington's Archbishop James A. Hickey had been seen as a particularly promising candidate. He not only heads the church in the nation's capital, but has taken a leadership role in the American bishops' conference and served as their point man in opposition to escalating United States military involvement in Central America.

With yesterday's appointments, the number of U.S. cardinals who have not reached retirement age now totals five -- if you count Cardinal William W. Baum, now in a high Vatican post. Poland has an equal number, if you count the pope. Italy now has 37.

Observers of the church in this country have futilely scanned successive lists of new cardinals for the name of Archbishop Jean Jadot, appointed apostolic delegate to this country by Pope Paul VI.

During his seven years here, Jadot's role in naming 174 bishops, most of them younger men of the activist and socially progressive mold, did much to change the face of Catholicism in this country. The "Jadot bishops," as they came to be called, made possible such landmark developments as the 1983 pastoral letter condemning nuclear war and the comparable work on the economy, now under way.

Five years ago Jadot was transferred by Pope John Paul II to head the Vatican Secretariat for Non-Christian religions, from which he retired a year ago. He was the first apostolic delegate to this country, a post considered near the top of Vatican diplomatic service, not to have been rewarded with a cardinal's hat.

Yesterday, Jadot's successor at the secretariat, Archbishop Francis A. Arinze of Nigeria, was named a cardinal.

Twelve of the 28 new cardinals appointed yesterday are Vatican bureaucrats or in the diplomatic service. At the Vatican as in the military, rank speaks to rank. Since several consistories in recent years have focused on churchmen from the Third World, the emphasis this time on church bureaucracy was seen as a catch up.

One name on yesterday's list has a special poignance, that of Archbishop Andrzej Maria Deskur of Poland, president emeritus of the Pontifical Commission for Social Communications. The day after he was elected pope, John Paul II left the Vatican for a Rome hospital to pray at the bedside of his longtime friend Deskur, gravely ill from a stroke.

Although Deskur was left severely disabled, the pontiff refused for a long time to replace him in the key communications post. Even after he was succeeded by Archbishop John Foley of Philadelphia, Deskur, in retirement, remains influential at the Vatican as one of the surest routes to the papal ear.

A surprise appointment was the Rev. Pietro Pavan, listed simply as "a church sociologist." Pavan, said to be over 80, is a former rector of Rome's prestigious Lateran University. He was characterized by Msgr. George Higgins of Catholic University as "one of the leading experts in Catholic social teaching."

A close friend of the late John Courtney Murray, Pavan was said to have written with Murray the Second Vatican Council's critically important document on religious freedom, which repudiated the church's long-held doctrine that "error has no rights."