The president of the National Conference of Catholic Bishops set a new political agenda for the nation's 50 million Catholics yesterday, urging stepped-up opposition to the nuclear arms race and abortion and a battle to retain government aid programs for the poor.

In his first presidential address as head of the nation's Catholic hierarchy, Archbishop John R. Roach of St. Paul-Minneapolis set those as the top priorities in a message devoted entirely to a call for political involvement based on moral convictions.

"From the debate on abortion to decision-making about Poland, from care of the terminally ill to the fairness of budget cuts, the direction our society takes must include an assessment of how moral and religious convictions relate to the technical dimensions of policy," he said.

The presidential address by the head of the hierarchy is roughly analagous to the State of the Union message of the president, charting directions for the year ahead. While the bishops' conference, made up of 270 prelates, has adopted numerous positions on political issues over the years, veteran church observers could not recall a previous presidential address focusing so insistently on the need for church involvement in public decision-making.

In other developments yesterday, Roach read a message from Pope John Paul II granting a request to change a key portion of the liturgy of the mass that many considered sexist. The change comes in the solemn prayer instituting holy communion, which will now affirm that Christ "died for you and for all" instead of the previous rendering, " . . . for you and for all men."

The change, approved and submitted by the conference last year, is to go into effect immediately.

Many priests, sympathetic to feminists' complaints, have already made the change in wording without waiting for Rome's approval.

Archbishop Rembert Weakland of Milwaukee, retiring head of the conference's liturgy committee who wrestled with two annual sessions of the bishops before securing the change, said that other requested liturgical changes were "still on the desk" at the Vatican, awaiting approval.

The bishops adopted a $20 million budget for their national office, which has headquarters here. The figure included an increase in their government liaison office, which handles lobbying for the church, from $325,370 to $397,935.

Bishop Thomas C. Kelly, general secretary of the hierarchy, called the lobbying unit "one of the offices which has the highest priorities" and added that "if necessary, its funding will be increased."

In his presidential address, Roach said, "On a global scale, the most dangerous moral issue in the public order today is the nuclear arms race," adding, "The church in the United States has a special responsibility to address this question, a responsibility underscored by Pope John Paul in his remarks at the White House in 1979."

Individually, nearly a score of Catholic bishops have issued statements denouncing the nuclear arms race. Roach called their actions "both prophetic and profoundly important." A report scheduled for this afternoon from Archbishop Joseph L. Bernardin of Cincinnati, chairman of the conference's committee on war and peace, is expected to address this subject further.

Roach linked expenditures on armaments to cutbacks in government programs for the poor. "The proposed expenditure of $1.5 trillion for defense over the next five years stands in stark contrast to budget cuts which threaten the food, the health care and education of the poor," he said. "What is spent for guns directly reduces what is available for the quality of care and life for the least among us."

The archbishop said that private charities cannot fill the gap created by cuts in government-funded programs. "We have neither the resources, nor, I suppose, even the mandate to do this. We will do our part, but our own social teaching calls upon the state to do its part."

Calling the 1.5 million abortions he said are performed annually in this country "a scandal," Roach said, "It is time to say, enough." He called for support of the Hatch Amendment, now before the Senate, which would recognize the rights of states to ban abortions.

At the same time, he added, legislation "is not the total solution to the evil of abortion. We remain committed to the proclamation of the Gospel message concerning sanctity of human life, and to the practical steps and programs required to eradicate the conditions which cause some to turn to abortion as a solution to personal or social problems."

Roach took issue with those who would argue that the principle of church-state separation precludes political activity. "The right of religious organizations of varying views to speak must be defended by all who understand the meaning of religious liberty and the social role of religion," he said, adding that he defended the right "of the Moral Majority or any religious organization to address the public issues of the day . . . ."

"I agree fully with the principle of separation of church and state," he said. "I do not agree that absence of dialogue about and between religion and politics serves either the church or the state."