The nation's Roman Catholic bishops are widely expected to lift tomorrow the automatic excommunication of Catholics who divorce and remarry.

The action will be taken in executive session by the National Conference of Catholic Bishops, which began its spring meeting here today.

A high conference official, who declined to be identified, said there was "widespread support" for rescinding the harsh penalty, which one bishop characterized privately as a case of the American bishops being "more Catholic than the church."

Automatic excommunication, which was mandated by the American hierarchy in the Third Plenary Council of Baltimore in 1884, does not exist in the Catholic church in other countries.

"Excommunication makes a person a "separated Catholic'," the Rev. Thomas Kelly, general secretary of the hierarchy, explained at a press briefing, "and deprives him of the prayers of the church, the services of the church (including the sacrament of holy communion), and the right to serve in certain capacities such as being godparents."

Kelly stressed that removal of the excommunication decree, if it is approved, would not affect church teaching on the indissolubility of marriage, which he called "unbreakable. There may be no doubt about that."

But he said lifting the automatic excommunication would "symbolize the church's lasting concern" for Catholics in tangled marriage situations. He said it would also encourage such persons to pursue efforts to have their marital status regularized in the eyes of the church.

The Catholic church holds that a "valid" marriage may be terminated only by the death of one of the partners. However, church marriage tribunals in recent years have greatly expanded the grounds on which a failed marriage can be found "invalid" and a church annulment can free the parties to remarry.

If the bishops act favorably on lifting the automatic excommunication, as expected, their action must still be approved by the Vatican, because the original 1884 action was also ratified by the Vatican.

The bishops' meeting opened today amid mounting tension over how the hierarchy will respond to recommendations of last October's Call to Action Conference concerning reforms in both church and society.

The conference climaxed two years of nationwide hearings to elicit the "vox populi" - the voice of the people.

In their draft statement of response - presented today for a vote on Thursday - the bishops rejected pleas for changes in the church stance on such controversial issues as birth control and the ordination of women or of married men to the priesthood.

The controversy has attracted partisans of various causes to a degree not seen since the antiwar years. Last night and today, between 40 and 50 demonstrators sang songs and held candlelight vigils on the busy downtown streets outside the Palmer House meeting.

When the bishops' sessions got under way this morning, clusters of plainclothes security men were present in the sixth floor hallways outside the meeting room. There have been no clashes.

In his opening address, Archbishop Joseph L. Bernardin, president of the hierarchy, took note of the "unusual pressures" in the church and among the bishops themselves over the Call to Action process, but pleaded for the bishops to preserve the unity of the church while dealing with their differences.

Acknowledging that "polarization and factionalism . . . are realities in the church at large," Bernardin said that, "because they do exist, we need to strive especially to affirm and effect unity among ourselves."