Catholic bishops made headlines across the country this week by issuing the draft of a pastoral letter calling the nation's 15 percent poverty rate a "social and moral scandal," and urging economic and welfare reforms to close the gap between rich and poor. If the Reagan landslide was based on peace and prosperity, perhaps the letter signals a logical next step: namely, that a peaceful and prosperous country will once again attend to its weakest members.

That, at least, is what the bishops are calling for.

A week before the election, the Center on Budget and Policy Priorities issued a report that describes nearly half of society's weakest members as living in female-headed households, and summarizes what has happened to them during the past four years. The center is a nonprofit, nonpartisan organization, supported mainly by the Ford and Field foundations.

Its report, based on analysis of data from the Census Bureau, the Bureau of Labor Statistics, the Congressional Budget Office, the Joint Committee on Taxation and other sources, concludes that "the status of female-headed households today . . . is worse than has been commonly believed."

More than 16.5 million people living in female-headed households in 1983 were living below the poverty line. More than half of all minorities living in such households and more than half of all children living in them are poor. The face of poverty, in overwhelming numbers, is that of the single mother and her children.

"Female-headed families have lower incomes and higher poverty today than in 1980," the center report concluded. "No other group in the population has experienced so sharp a drop in its standard of living as these families have." It cited as backup an Urban Institute study that found female-headed families suffered an average loss of $400 each in disposable income in 1984 compared with 1980, with the poorest of these families being especially hard hit with a loss of $600 apiece. The income loss hit female-headed families in every income strata -- those with more than $25,000 in income lost 1 percent, even with the drop in inflation -- while two-parent families on the average came out ahead.

The institute blamed the loss directly on Reagan administration tax and budget policies and found that had they not been followed the income loss would have been less than one-sixth as much.

"The perception is that people at the lower income levels aren't doing okay but that the richest female-headed households are," said David Kahan, a policy analyst who wrote the center's report. "But they're losing out also. That's a fairly alarming result. The problems of female-headed households are significant enough to begin with. To think you can't make it as a female-headed household even if you're better off portends pretty negative things for society."

One of the most striking impacts of the tax policies, as analyzed by both the Urban Institute and the Congressional Budget Office, was that households with incomes below $10,000 would lose an average of $1,100 each between 1983 and 1985 as a result of budget and tax policies, while those with incomes over $80,000 -- less than half of 1 percent of which are headed by females -- would gain an average of $24,000 each.

The center found that not only were female-headed households "disproportionately affected by the budget cuts, but in many cases their taxes have gone up sharply as well." For example, a family of four at the poverty line that paid $460 in taxes in 1980 will pay $1,076 today. As a result of the 1981 Economic Recovery Tax Act, the tax burden for the bottom 40 percent of the population -- where most female-headed households are found -- has risen, while the taxes on the affluent and on corporations has dropped.

The Urban Institute found that the sharpest increases fell on the bottom 20 percent, where nearly half of all female-headed families are clustered. "As a result," the center noted, "tax burdens have effectively been shifted from male-headed families to female-headed families as well as from rich to poor."

The draft of the bishops' pastoral letter has served to bring the issue of poverty among plenty to the fore, an issue that the Reagan administration has tried mightily to ignore. The administration has said, however, that tax reform is at the top of its list. It is incumbent on those concerned about poverty, be they bishops or members of Congress, to ensure that the issues of tax reform and its impact on female-headed households be joined.