Daniel Patrick Moynihan, who is either the best social scientist serving in the Senate or the best senator who is also an active scholar, has just published a paperback edition of his 1985 Godkin Lectures at Harvard, ''Family and Nation.'' An epilogue reviews the current state of debate on the linked subjects of children, poverty and government policy.

Twenty years after his study for the Johnson administration of the forces undermining family stability in the black community ignited a huge controversy, Moynihan cites evidence showing that the pattern of illegitimacy and divorce has now become epidemic in the general American society.

More than one-fifth of the nation's children are in single-parent families. Half the children now being born will live in such families at some time before they are 18. The shaky earnings and anemic public assistance for these families mean that, for the first time in American history, poverty is afflicting children more than elderly persons.

These and other points made by the New York Democrat have struck a responsive chord in a country that had taken a holiday from social policy concerns while it was recovering from the severe inflation of the 1970s. President Reagan ordered up a study of ''family policy.'' States from Massachusetts to California developed successful pilot programs for helping single mothers move off welfare into productive employment. And for a time, it appeared there might be bipartisan agreement on a national ''family policy,'' ''welfare reform'' or ''welfare prevention'' strategy.

As this session of Congress winds down, preoccupied by unresolved budget, tax, foreign policy and Supreme Court battles, it is apparent that those hopes will not be realized in 1987. Whether sufficient political energy can be mustered in presidential campaign year 1988 is problematic.

A welfare reform bill crafted in the House Ways and Means Committee has stalled because of concern over its cost and the threat of a presidential veto. More modest Senate legislation, drafted by Moynihan, has been criticized by the White House because it would make the basic Aid to Families with Dependent Children stipends available everywhere to two-parent families without jobs, thus (in the White House's distorted view) ''increasing the welfare load.''

But the outlook is not entirely bleak. As Isabel Sawhill of the Urban Institute, a Washington think tank, notes in a new report, ''The welfare system is but one possible means of alleviating poverty in the United States . . . the last, and not the first, line of defense in the fight against poverty.''

In the epilogue to his new paperback, Moynihan writes that ''arguably . . . the most important anti-poverty legislation since the Economic Opportunity Act of 1964'' was a provision in last year's tax reform bill. It doubled the personal exemption for each family member to $2,000, raised the standard deduction and indexed them to future inflation.

The effect of these changes was to stop taxing families of the working poor back into poverty. Through an arcane feature called the earned-income tax credit, it is even possible for the government to send a check of $500 or so to help a single parent with one child and an income of $10,000 a year.

Moynihan points out that these measures, which ''took off the tax rolls the six to seven million persons we were then taxing into poverty,'' were, remarkably enough, the least controversial parts of the whole tax bill.

To underline that point, Rep. Jack Kemp of New York, perhaps the most conservative of the Republican presidential contenders, was praising the same tax law provisions for the same reasons in speeches he gave last week. Kemp offered the useful idea that the exemption be doubled again for each child a family adopts -- as an incentive to reducing the large numbers of the children of poverty now living in institutions or foster homes.

In the present climate, neither Democrats nor Republicans are comfortable talking openly about ''income redistribution.'' But they are increasingly ready to acknowledge the painful reality of poverty in American life.

The massive voter survey the Gallup organization released two weeks ago under the auspices of the Times-Mirror Company suggests that ''social justice'' may be the dominant issue of the 1988 campaign. Presidential candidates from Bob Dole to Jesse Jackson are talking about their own families' experiences in the welfare system and their struggles with poverty.

If the 100th Congress does not come to grips with the problems Moynihan outlines in ''Family and Nation,'' a new president and Congress may be ready to move in 1989.