The nation's Roman Catholic bishops next week will begin a public campaign to make a moral issue out of the condition of the poor in this country and abroad.

A draft of a proposed pastoral letter on the economy was mailed yesterday to the 290 U.S. bishops and will be presented next week to the meeting of the National Conference of Catholic Bishops here.

The letter, written by a committee of five bishops after a year of hearings and debate within the church, is intended to raise the issues of economic equity and "fairness" that Walter F. Mondale tried -- and failed -- to run on in his overwhelming loss to President Reagan in Tuesday's election.

The drafting committee, headed by Archbishop Rembert G. Weakland of Milwaukee, has kept the contents of the letter closely guarded and delayed its release until after the election, to avoid charges of political motivation.

Nevertheless, the bishops' efforts are likely to prove as controversial as their stand against nuclear war, taken in the pastoral letter on war and peace released in May 1983.

The preparation of the letter on economics already has stirred a reaction among a group of prominent Catholic business executives and academics, headed by former Treasury secretary William E. Simon, which released its statement yesterday, in advance of the bishops' letter.

The statement by the Lay Commission on Catholic Social Teaching and the U.S. Economy praises the ability of America's economic system to create and spread wealth among its citizens and cites a religious justification for freedom of choice and the other "institutions of a free economy."

After its formal release next week, the bishops' letter will be discussed throughout the church, and, if the bishops agree, may be finally adopted a year from now.

The letter will question whether an economic philosophy rooted in individual achievement and a minimal role of government is in tune with the moral teachings of the church, according to some informed Catholic clergy and laymen. Some Catholic commentators have expressed concern about an apparent erosion of public support for programs that help the poor and disadvantaged.

"My understanding of the letter is that the bishops will be asked to examine the effects of the American economy with respect to its adequacy in meeting human needs . . , " said Bishop James W. Malone of Youngstown, Ohio, president of the National Conference.

A central theme of the bishops' effort is the necessity of giving "a preferential option to the poor" -- a concept embraced by Pope John Paul II during his meeting with Latin American bishops at Puebla, Mexico, in 1979.

That means trying "to see things through the eyes of those who are are hurting or oppressed in any way, rather than through the eyes of those who are wealthy or successful," said Bishop Peter Rosazza, auxiliary bishop of Hartford, Conn., one of the economic letter's initiators.

Church members familiar with the bishops' letter said it contains an opening section discussing the religious values that should be woven into economic policy decisions.

The remainder of the letter deals with four or five specific economic issues: employment, poverty, economic planning and trade with developing nations, and food and agriculture may also be discussed, the church members said.

The opening will explain that the bishops are speaking out on the issue because they think the economy should not be judged just on how efficient it is, but also on how it affects people, church sources said.

The letter includes an analysis of the Bible's teachings on economic issues, with emphasis on the common good rather than the achievement of individuals. "The biblical section also explains the theological reasoning behind the church's preferential option for the poor," according to Thomas J. Reese, an associate editor of American magazine, who has closely followed the bishops' work.

It also includes a historical perspective on the church's stance on economic issues, reaching back to the first modern social encyclical, Rerum Novarum, issued by Leo XII in 1891, which upheld the right to hold private property but said it should be used for the common good of all people. The encyclical also endorsed labor unions and rejected Marxist ideas about class conflict.

The most closely held aspects of the letter are the comments on economic policy, which apply religious values to the four or five economic policy areas. The letter is expected to stress government's role in providing employment for the needy through publicly financed jobs and support for those unable to work.

It will discuss the disruption to the lives of factory workers and their families from the decline in manufacturing employment and the growth of a service economy, with government job training as an essential response, according to Peter Steinfels, editor of Commonweal, an independent magazine on religious and political issues.

It is unclear how far the bishops will go in discussing international trade, foreign aid, and the Third World debt problem.

Bishop Weakland's drafting committee also backed away somewhat from its initial support for strong governmental planning in the economy, Reese said. During the year of hearings, the bishops also recognized the troubling tradeoffs and conflicts that confront policy makers who seek to help the poor and lower-income people, he added.

A policy that helps unemployed Southern garment workers by raising import barriers, for example, hurts apparel workers in less developed countries. A policy of using government spending to spur growth in the economy may also increase inflation -- hurting those who are meant to be helped, Reese said. "They recognize this. We'll have to wait until [today] to see how [the letter] is phrased. I think it will be more on the side of the developing nations than the AFL-CIO, which will be very tough for the bishops. They've been very pro-labor."

The bishops also have been conscious of the debate their plan will spark within the Catholic church.

That debate was underscored by the statement issued yesterday by the Lay Commission, whose members include J. Peter Grace, chairman of W. R. Grace & Co.; William M. Ellinghaus, retired president of American Telephone & Telegraph Co., and former secretary of State Alexander M. Haig Jr..

"If you want to meet the needs of the poor, you have to have invention and investment by the rich and everyone else who can help out," said Michael Novak, a Catholic social thinker now at the American Enterprise Institute here.

"We're asking what political economy has ever been better" for the poor, for immigrants and the disadvantaged, said Novak. "The one revolution that works is the liberal revolution of democracy, capitalism and pluralism."

Government has a responsibility to help care for the disabled, elderly and ill who are far enough removed from the economy that the normal creation of wealth cannot help them, the lay panel said.

The implicit disagreement with the bishops' letter seems to center on the degree of aid the poor and the displaced should get even at the cost of some economic efficiency. The lay group mentions that individuals who lose their jobs need particular help with medical insurance and mortgage payments, two types of aid not at all common today.

But their statement never addresses the difficult problem of where the line between equality and efficiency should be drawn, nor does it suggest how to go about addressing the issue.