For many of the nation's big cities, the preliminary 1990 census figures tell a depressing story: greater than expected population losses that will result in a weakened political base and, for those who remain behind, increasingly grim and isolated social conditions.

In Chicago, for example, the census reported population declines of more than 9 percent since 1980, a trend tied to a devastating loss of manufacturing jobs. More than 60 percent of the remaining population is minority, with 40 percent of families living in poverty in many inner-city neighborhoods and 18 percent of black residents unemployed.

In Baltimore, a population loss of nearly 67,000 presages a significant shift of power as the predominantly black city's political districts are extended into the mostly white suburbs. Among others, the changes will affect Rep. Kweisi Mfume (D-Md.), whose district falls far short of the required population, a situation also true in 19 other of the 24 congressional seats held by blacks, according to the Joint Center for Political Studies.

In Washington, population figures show a nearly 10 percent decline to below 600,000, while many of the surrounding counties in Maryland and Virginia experienced double-digit growth.

But the most stunning decline was registered in Detroit, where the population figure fell 19 percent to 970,000. Because Michigan law gives special taxing authority to cities with populations of 1 million or more, the new figure meant Detroit could lose its authority to levy a utilities tax and be forced to lower its income tax rate.

And even after several decades of migration to the Detroit suburbs, the slide below the 1 million figure came as a psychological slap to the city, whose officials have been scrambling to find households missed by the Census Bureau.

"This is a long-term trend that's not new . . . but it jarred people," said William Frey, a researcher at the University of Michigan's Population Studies Center.

While metropolitan areas have grown rapidly during much of the 20th century, many of the cities at their heart began losing population in large numbers 20 years ago. The steepest drop took place during the 1970s, when several central cities suffered population losses of 20 percent or more.

Demographers expected to see a continued loss of population for many of these cities in the 1990 numbers, but the extent of the falloff surpassed even the pessimistic estimates.

The preliminary figures released late last month are likely to change as local governments challenge the findings and other segments of the population are tallied. But demographers and social scientists say that even the early figures will have wide political and social repercussions in many big cities.

"We are going to be an overwhelmingly suburban society with tremendous social crises in the central city," said Gary Orfield, a political science professor at the University of Chicago. "It's going to make a bad situation worse."

Behind the tabulations is a complex web of cause and effect. As middle-class residents leave for the suburbs, urban centers shrink and the tax base declines, leaving the city fewer resources to deal with the additional demands of the lower-income, less-educated population that remains.

While black families have joined the exodus, they have done so less frequently and for a shorter period than whites, meaning that the population remaining behind and bearing the brunt of the decline is predominantly minority.

And at a time when these communities will most need state and federal assistance, they will wield the least political clout. State legislative seats will continue to move away from the cities to the suburbs, and political scientists warn this will mean a corresponding shift of attention away from issues of concern to the urban centers.

At the congressional level, most seats now held by blacks are expected to remain in the hands of blacks. But as the districts are redrawn in response to the 1990 census, candidates vying to represent those jurisdictions will have to adjust to a new, more suburban constituency.

"Black candidates are going to have to appeal to a broader audience, a white audience," said Reynolds Farley, a demographer at the University of Michigan.

Mark Gersh, Washington director for the National Committee for an Effective Congress, said a population loss of 60,000 in St. Louis, for example, will mean DemocraticRep. William Clay's district will probably not remain majority black when it is reconfigured.

Earlier population estimates indicated that half of the 10 congressional districts expected to suffer the greatest population losses were represented by blacks, said Milton Morris, director of research at the Joint Center for Political Studies.

Heading the list were the Michigan districts represented by George W. Crockett Jr. (D) and John Conyers Jr. (D) and the Philadelphia district represented by William H. Gray III (D), each of which will have to gain at least 100,000 people when it is redrawn.

Many city officials argue the new figures bolster their argument that the Census Bureau failed to find millions of the nation's residents in its once-a-decade headcount. But most social scientists say that these numbers primarily reflect a very real movement of population.

"The out-migration is going to be more important than the under-count" in explaining the figures, said Bill O'Hare, a demographer at the Population Reference Bureau.

Ron Mincy, an economist at the Urban Institute, said an undercount could explain some of the loss, but that the numbers also reflect the movement of working-class blacks away from the cities in search of jobs in industry.

"What we're seeing in the 1990 census is a continuation of what we saw in the '80 Census," he said, describing a "depopulation" of the worst city neighborhoods in the 1970s.

These "worst neighborhoods," defined by their high rates of unemployment, welfare dependency, high school dropouts and female-headed households, lost population by an average of 23 percent during the 1970s, compared to an average of 2 percent in all neighborhoods across the country, Mincy said.

"The areas that appear to be losing population are the same areas that were losing population then and they haven't turned around," he said.

Population losses were registered in half of the 20 largest cities, including New York and Philadelphia. And there were declines in 10 of the 11 cities with large black populations.

Those cities, where populations number at least 100,000 and were at least 49 percent black in 1980, include Gary, Ind., Atlanta, Newark, Birmingham, New Orleans, Richmond and Savannah, Ga., as well as Washington, Baltimore and Detroit. Only Inglewood, Calif. registered a gain.

In many cities, officials are preparing to challenge the preliminary census numbers in an effort to preserve the federal funding and political power tied to the results. Local governments have until late this month to produce their own utility records, tax files or other documents to prove that census workers missed some households.

This process of local review could boost city numbers substantially, but is unlikely to turn around the general population trends.

Those trends stem from a continued loss of manufacturing employment, particularly in the Midwest. Over the 1980s, the country lost 2 million manufacturing jobs, said Wayne Vroman, an economist at the Urban Institute.

Baltimore, for example, has seen its manufacturing base cut in half, from 100,000 to 50,000 jobs, over the past two decades, Vroman said.

"The clearing out is a real phenomenon and much of it is associated with manufacturing declines in central cities," he said.

Those changes are at the heart of some of the most troubling demographic trends in the nation, say social scientists.

Sociologist William Julius Wilson noted in a recent address that most of the rise of poverty in American ghettos was located in the large industrial metropolises of the Northeast and Midwest. Half of the increase in ghetto poverty over the 1970s had taken place in only two cities -- New York and Chicago.

"Poverty in the United States has become more urban, more concentrated and more firmly implanted in large metropolises," he said, "particularly in the older industrial cities with immense and highly segregated black and Hispanic residents."