The U.S. Catholic bishops, resisting administration pressure to soften their stance on production and use of nuclear arms, yesterday added a possible condemnation of the MX missile to a proposed pastoral letter on the morality of nuclear warfare.

A second draft of the U.S. Catholic hierarchy's two-year effort to apply Christian morality to the nuclear age in some ways strengthens the anti-nuclear strictures contained in the first draft issued last June.

The new document reaffirms the bishops' opposition to several basic elements of U.S. nuclear strategy, such as the threat of first use of atomic weapons if necessary to repel a Soviet invasion of western Europe. It rejects any use of atomic weapons against military targets if populated areas are nearby or against populated areas even if American cities have been struck first.

Retention of these positions suggests that letters written by Defense Secretary Caspar W. Weinberger and White House national security adviser William P. Clark over the summer raising questions about the first draft had little discernible effect.

The new draft also opposes "the willingness to foster strategic planning which seeks a nuclear war-fighting capability." Both the Carter and Reagan administrations have tried to figure out how an atomic war might be fought if deterrence fails; critics complain this reflects a belief that nuclear war is survivable, and thus thinkable.

The document raises questions about the role of Catholics in the U.S. armed forces in carrying out certain elements of atomic strategy and also about workers in the defense industry who produce weapons of mass destruction.

The bishops issued a strong call for arms control, supporting "immediate, bilateral verifiable agreements" between the United States and the Soviet Union to halt the testing, production and deployment of new weapons, and then negotiated bilateral "deep cuts" in the arsenals of both superpowers.

The bishops do not recommend a course for reaching such agreements. The administration argues that the only way to negotiate with the Soviets on arms is to build up U.S. strength first.

The bishops also voiced support for a comprehensive ban on nuclear testing, on which the Reagan administration has deferred action.

"We do not perceive any situation in which the deliberate initiation of nuclear warfare, on however restricted a scale, can be morally justified," said the second draft of the statement which the church's 260 bishops will debate in their annual meeting here next month.

The new 110-page draft, which in final form will become the basis for the 50-million-member U.S. church's teachings on the subject, said nuclear deterrence "may still be judged as morally acceptable, provided it is used as a step toward progressive disarmament," but called for "continual public scrutiny of what our government proposes to do with the deterrent."

This means that "each proposed addition to our strategic system or change in strategic doctrine must be assessed . . . in light of whether it will render . . . arms control and disarmament more or less likely," the document said.

The draft statement warned especially against "the development and deployment of destabilizing weapons systems on either side" and "weapons which are likely to invite attack and therefore give credence to the concept that the U.S. seeks a first-strike, 'hard-target kill' capability."

It then went on pointedly to note, as the first draft had not, that "the MX missile might fit" in this category.

The bishops' draft statement is the work of a six-member committee, including hawks and doves of the hierarchy, headed by Archbishop Joseph L. Bernardin of Chicago.

The bishops will debate the second draft next month but postpone a final decision until a special meeting in Chicago next May.

The timing of next month's debate, particularly if it involves the MX missile, could be a problem for the administration because President Reagan has told Congress he will finally recommend a basing system for the controversial intercontinental ballistics missile in December.

The MX missile has been promoted by the administration as one of the main new weapons to be added to the U.S. strategic arsenal over the next two decades. But there are longstanding differences over how to base it so it will not be vulnerable to attack.

In the Catholic Church, pastoral letters from the hierarchy are advisory and hortatory rather than binding on individual church members. But faithful Catholics are supposed to take seriously the bishops' advice in forming their consciences and courses of action.

The amount of time and effort involved in this pastoral, plus the gravity of the subject matter, is expected to assure it serious consideration. While several popes have issued pronouncements on the nuclear question, this is the most comprehensive effort of any individual or group in the Catholic Church to apply the age-old "just war" theory of St. Augustine to the complexities of the nuclear age.

Copies of the document have been sent to Catholic leaders in countries where nuclear warfare is an issue, Germany, England, the Netherlands, France, and to the Vatican. A spokesman for the U.S. bishops said the latter "commented at some length" on the first draft but "did not have any great problems with the document."

The nuclear debate by Catholic leaders in this country parallels similar concerns in most Protestant communions in this country and in western Europe.

The draft released yesterday speaks repeatedly of the church's role in helping to mold public opinion "to resist resort to nuclear war as an instrument of national policy." It says "there should be a clear public resistance to the rhetoric of 'winnable' nuclear wars, 'surviving' nuclear exchanges and strategies of 'protracted nuclear war.'

"We seek to encourage a public attitude which sets stringent limits on the kind of actions our government will take on nuclear policy in our name," the statement says. "We believe religious leaders have a task in concert with public officials, analysts, private organizations and the media to set the limits beyond which our military policy should not move in word or action."

A section on workers in defense industries notes that because nuclear weapons as deterrents may be tolerated "while meaningful efforts are under way to achieve multilateral disarmament . . . we cannot at this time require Catholics who manufacture nuclear weapons" to quit their jobs if they believe their work reduces the likelihood of war.

But it adds: "Should we become convinced that even the temporary possession of such weapons may no longer be morally tolerated, we would logically be required to consider immoral any involvement in their manufacture."