A panel of national Democratic leaders, sounding angry moral appeals reminiscent of the social activist era of the 1960s, today said the federal government must mount a national effort to break the "self-destructive" behavior of the urban black underclass.

In a forum here hosted by New York Gov. Mario M. Cuomo and led by former Virginia governor Charles S. Robb, Sens. Daniel Patrick Moynihan (N.Y.) and Sam Nunn (Ga.), House Budget Committee Chairman William H. Gray III and others, the Democrats were quick to concede that they are "grievously short of ideas," as Moynihan put it, on how to proceed.

They likewise acknowledged that even if they were able to fashion a new social policy for the poor, they would not be sure, in a time of $200 billion annual federal budget deficits, if they could craft the political consensus to pay for it.

But the session was notable nonetheless for the fervor with which an ideologically diverse group of Democrats -- some of whom are thought to harbor presidential or vice-presidential ambitions for 1988 -- all said their party must bring the underclass to the center of the national political and policy debate. It was also notable for the rejection of the notion -- prevalent among conservative social thinkers -- that government is more cause than cure of the pathologies of the poor because it fosters a culture of dependency among recipients of its welfare.

"We need to launch a new targeted offensive against joblessness, poverty and dependency," said Robb, chairman of the Democratic Leadership Council, a self-styled "centrist" group of party elected officials that cosponsored today's forum with the New York State Democratic Party.

"Government has a deterrent and curative role" in eliminating the underclass, Cuomo said. "Now the question is which programs should be preserved and which should be eliminated." While Cuomo took the Reagan administration to task for making the "denial of concern acceptable, perhaps even respectable," Moynihan trained his fire on fellow Democrats.

"It has become a belief, bordering on prejudice, that the social ills of the present are the consequence of misguided Democratic social policies of the past two generations," he said. "None hold this belief more guiltily, if furtively, than the Democrats themselves."

He said his party is in its "worst shape since the immediate aftermath of the Civil War . . . . The parallel is not perfect. The Copperhead Democrats of the Civil War never repented. The veterans and inheritors of the New Deal and Great Society do little else."

Moynihan's vision of the problems of the black urban poor, a subject he has studied for decades, was more stark. "In many of our major cities, we are facing something like social regression," he said, reciting a now-familiar list of ills that includes teen-age pregnancy, births out of wedlock, drug use and crime. Moynihan called these social ills "post-modern" and said they reflect a far more "self-destructive" pattern than anything affecting the poor of earlier generations. "I grew up in Manhattan not far from here in an area known as Hell's Kitchen," he said. "By today's reckoning, it was a peaceable kingdom."

Moynihan said conservatives who say self-reliance is the only way to confront these ills have a point -- but only a point. "By all means let us go on about self-reliance, gumption and go-gettingness," he said. "But if that is all there is to be by the way of social policy, no one needs Democrats. And if that is all the social policy there is to be, Democrats will have earned their eclipse."

Glenn C. Loury, a professor of political economy at Harvard University's Kennedy School of Government and a leading conservative black scholar, argued that while the availability of welfare may not cause self-destructive social behavior, it "underwrites" it. He said what is needed is a restoration of values in the ghetto, and that this is best brought about through community structures such as churches and civic groups.

Gray responded that when he grew up in all-black North Philadelphia, "on every block there were poor people, but there were also doctors and working poor." In the past generation, as desegregation opened opportunities for the black middle class, he said, the role models and "mediating structures" have moved to middle-class and suburban communities -- where they do poor blacks little good.

Gray was not not prepared to commit to raising taxes to pay for a new urban policy -- saying presidents must lead in the tax area. He said, however, that recent budget votes in both chambers of Congress make it clear that the Reagan military buildup is over, and that there will no longer be any transfer from domestic to military spending.

Still, Moynihan was gloomy about finding money for a federal urban effort. "We face a protracted fiscal crisis for the next two or three presidencies," he said. "We have sold out this generation."