Mary Rose Oakar comes from a part of America that is in decline.

On her office wall is a picture of a white frame duplex that stands at the corner of Bridge and West 30th streets on Cleveland's West Side. It is the house she grew up in, the house where she still lives. Her world was and is typical of much of older, urban America -- a world of stable, tightly knit ethnic neighborhoods where the people worked hard, saw a better future for their children and voted Democratic.

That world is diminishing. In a highly mobile society, fewer and fewer Americans have chosen to stay so close to their childhood roots. Over the years more and more of the children of the West Side have sought their futures elsewhere. They still work hard, but when they vote, which is less often than their parents, it is not always Democratic.

These are matters of more than passing interest to Oakar. She grew up in that duplex at 30th and Bridge to become the congresswoman from the 20th district of Ohio. But now, because the West Side is diminishing, her political future is threatened, not by any potential Republican opponent, but by a brutal numbers game that is likely to pit her next year against another Democrat from a neighboring district.

According to the census figures, there are 87,538 fewer people in the 20th district today than there were in 1970, an almost 20 percent population loss over the last 10 years. The 20th, with 374,942 certified residents, is the country's 13th smallest congressional district, less than half the size of the largest, Florida's 5th district, which has a population of 880,078.

Sometime within the next year, the Ohio legislature will redraw the state's congressional district boundaries to conform with the population shifts measured by the 1980 census. In the last 10 years, Ohio was one of the places that stood still. The state's population, according to the census, increased by only 1.3 percent during the 1970s.

But to stand still while the rest of the country is growing by more than 11 percent is to fall behind, and Ohio is falling behind. The congressional redistricting that will take place in the next year will reduce the state's representation in the U.S. House of Representatives by two seats, from 23 to 21 districts. These are two of the 17 congressional districts that will shift from the Northeast and Midwest to the South and West as a result of redistricting.

Oakar's district will almost certainly be caught in this shuffle. For while the country as a whole was growing and Ohio was standing still, the 20th district was shrinking at an alarming rate.

Somewhere, more than 125,000 new people must be found for Oakar to represent if she is to come close to qualifying under the Supreme Court's one-man, one-vote edict.

Those new people will not be found in the old neighborhood around 30th and Bridge. Instead, they will have to be found further out in Cuyahoga County, probably in what is now the 23rd Congressional District, itself some 50,000 people short of the mandated size for an Ohio congressional district under the new population figures.

The 23rd district is represented by Ronald M. Mottl, like Oakar a Democrat. When the redistricting process is over, local political figures here expect, one of them will not be in Congress. Cuyahoga County, which 10 years ago numbered more than 1.7 million people and today has a population of 1.5 million, is no longer big enough for both of them.

Oakar and Mottl have this one, small comfort about their predicament: they are not alone.

All across the great urban industrial belt that stretches from Chicago east to the Atlantic Ocean, the story, and the numbers, are much the same. It is not just the big cities that are losing population; the older, established suburbs around cities like Cleveland are also in decline as people, seeking cheaper housing and a better environment, have moved further out into the country.

Meanwhile, industry, in search of a more favorable tax and business climate, has shifted thousands of jobs to the South and West, compounding the problems of the Northeast and Midwest.

The city of Cleveland and the state of Ohio suffered as much from this as anyplace in the country. Between 1970 and 1980, for example, there was an 11 percent decline in manufacturing jobs in the state, costing Ohio 154,000 jobs.

Many of these lost jobs were in the Cleveland area, which thrived when its basic industries -- steel, automobiles, tool-making -- did well and which has suffered as those industries have gone through hard times. And as the jobs in manufacturing and heavy industry have declined, they have not been replaced in sufficient numbers by jobs in new, high technology industries that are growing, but growing in other parts of the country.

Politically, the Democrats are the losers in this process. It may be true, as Ann Lewis, the political director of the Democratic National Committee, argues, that Democrats will more than hold their own in the growing areas of the so-called Sun Belt.

But that will not alter the fact that areas that for decades have formed a solid Democratic base are shrinking. When the next Congress convenes in 1983, the delegation from Cuyahoga County almost certainly will be entirely Democratic, as it was throughout the 1970s. It's just that it will be made up of three Democrats instead of four.

Population figures compiled by Eric A. Weld Jr., professor of urban studies at Cleveland State University, document what has hapened to Cuyahoga County.

In 1950, Cleveland hit its peak population of 914,808, more than twice the 474,724 population of the suburbs within the county. The 1950s saw the beginning of the decline of the cities and the rise of the suburbs. The 1960 census recorded a drop in Cleveland's population to 876,050, and a huge growth spurt in the suburbs, which reached a population of 771,845.

This trend continued in the 1960s. The next census, in 1970, showed that for the first time more people (969,956) lived in the Cuyahoga County suburbs than lived in the city of Cleveland (750,879).

The 1980 census, however, recorded an entirely new phenomenon. Cleveland's population loss accelerated during the 1970s, with the number of city residents slipping to 573,822. But for the first time the census also showed a population loss in the suburbs, which declined to 924,473.

It was not just the West Side in the 20th district that was in decline. It was also places like Parma, Ohio, the heart of Mottl's 23rd district political base.

Where have all these people gone?

Despite the huge rise in gasoline prices during the 1970s, they have elected to move further and further from the city, bypassing long established blue-collar Cuyahoga County suburbs such as Parma, Brook Park and Brooklyn, Ohio. All but one of the counties that border Cuyahoga gained population in the 1970s. One of these, Medina County, grew by almost 37 percent, a growth rate that would qualify it for honorary Sun Belt status.

The elderly, given an option that was not available to earlier generations, have chosen to leave the snow and cold of Ohio's winters. "When I was a kid, anyone who went to Florida was a millionaire," said John Coyne, the mayor of Brooklyn. That is no longer the case, Coyne said, ticking off the names of friends who have retired to Florida.

As for the young, they have followed a familiar pattern, moving where the economic opportunities lie and serving as magnets for others to follow. Tim Hagen, the Cuyahoga County Democratic chairman, cites the case of his cousin, who 17 years ago moved with her physician-husband to Arizona. She has since been joined in Arizona by three sisters. All of them, Hagen noted, are now far different in political outlook from their father and his uncle, a steelworker from Whiting, Ind.

"That's what happens," he said.

Compounding these trends, according to Weld, has been the inability of places like Cleveland to provide the economic and other incentives that attract newcomers to the area. The key reason for the population decline in Cuyahoga County, he said, has not been too much migration out, but too little migration in.

"The problem here is immobility, not mobility," Weld said. "The city originally was a place that absorbed young people and poor people. When they made it in Cleveland, they moved out, but they were replaced by others. It was a beautiful system, but somewhere around 1965 it stopped working."

Jean Woodall came to Cleveland when the system was working. In the 1950s, she moved with her first husband from West Virginia to Florida. But there were no jobs to be had in Florida back then, so they moved on to Cleveland where her parents had preceded them and where her husband found a job 10 hours after arriving.

"That was when Cleveland was booming," she said.

But now Woodall, a volunteer worker for a neighborhood preservation organization on the West Side, is going back to West Virginia. She is moving, she said, so her children can escape the effects of a local school busing court order. Her mother and second husband are already back in West Virginia, and she knows of dozens of other families moving back to areas they had abandoned 20, 30 and 40 years ago.

"It's really sad," she said.

The political response to the population shifts recorded by the 1980 census will not work its way through the Ohio legislature until sometime next year, but the broad outline of what that response will be is already known.

Like so much of politics, it is likely to involve a compromise. If it were just a matter of numbers, it is only the Democrats who would suffer -- it was, after all, the two Democratic districts in Cleveland that suffered the most population losses.

As much as the Republicans would love to impose a two-seat loss on Democratic Cuyahoga County, they lack the power to do so. The governor, James Rhodes, is a Republican and the state Senate is controlled by the GOP. But the Ohio House is in the hands of the Democrats. Out of such divisions in state government are most redistricting compromises born.

The Ohio compromise, political leaders here believe, will involve a willingness by the Republicans to carve up the rural and Republican 17th district. The major factor in the GOP's willingness to surrender the 17th is that the incumbent Republican congressman, John M. Ashbrook, is set to make a run for the U.S. Senate next year.

The Democrats, in turn, will give up one of their safe seats in Cuyahoga County. Again, if it were simply a matter of numbers, the district to be sacrificed would be easily identified: it would be the 21st district, on the East Side of Cleveland, which has been represented since 1969 by Democrat Louis Stokes.

Stokes' district lost even more population than Oakar's during the 1970s and is now the seventh smallest congressional district in the country. The district was created in the mid-1960s to give Cleveland a black member of Congress. In the last decade, thousands of blacks in Cleveland, like blacks in other large cities, moved to the suburbs. In the redistricting of the 1980s, the lines of Stokes' district are likely to be redrawn to keep pace with the movement of his constituents and protect his political base.

"With a senatorial and gubernatorial election coming up next year," said Bob Hughes, the Cuyahoga County Republican chairman, "I can't see either political party being engaged in a conscious effort to eliminate the one black district in the state of Ohio."

Other factors, including friendship with the speaker of the Ohio House, will probably protect the political interests of first-term Democrat Dennis E. Eckart, who represents the suburbs east of Cleveland. So for both personal and political reasons, it is likely that it will come down to Oakar and Mottl, two Democrats thrown into one district.

The betting, at least for now, is that it will be Mottl, the more conservative of the two, a rare northern Democrat who voted with President Reagan on the budget battle, who will be left standing when it is all over.

All of this is naturally disconcerting to Hagen, the Democratic county chairman who worries about the Northeast becoming the nation's "inner city," filled with "the old, the poor and the sick." But even Hughes, his Republican counterpart in Cuyahoga County, sounds less than enthusiastic when discussing the shifts in population and political power that the 1980 census measured.

"Those parts of the country that tend to be more conservative are going to get more representation," he said. "And unless there is a failure of the economic policies of the Reagan administration, the conservative group that controls the Republican Party will grow stronger. I think this is going to create an enormous shift in the political attitude of the country as areas that tend to be more conservative get more clout."