If current migration trends continue to 1980, the northern states will lose at least 11 seats in Congress and the South and West will gain at least 10, the Census Bureau said yesterday.
In the East, only the state of Maryland, which now has eight members of Congress, will gain a seat if the recent population patterns hold, the bureau reported.
The biggest loser would be New York, whose 39-member delegation in the House would be cut by four - a drop of more than 10 percent. The biggest winner would be Florida, which now has 15 seats and would gain three under one population projection and four under another.
Yesterday's projections represent the first official indications of the political impact of the dramatic population shifts in the country that will be tabulated in the 1980 census.
Census sources stressed that the actual counts are virtually certain to differ from the projections and that very small population changes can affect a state's share of congressional seats.
The bureau's projections accompanied a census report projecting state populations between 1975 and the year 2000.
They indicate that if recent migration trends continue, the South and West will row two to three times as fast as the Northeast and North Central states during that period.
The bureau's projections fall into two categories, one based on migration patterns between 1965 and 1975, and the other on patterns between 1970 and 1975. The difference, basically, is that in the 1970-75 period, the northern regions lost population more rapidly and small towns and rural areas, especially in the West, gained population more rapidly than they did over the 1965-75 period. The South grew at a rate of about 15 percent in both periods.
If the five-year trend continues, the District of Columbia, which has 712,000 people in 1975, will have 627,000 in 2000, the bureau said.
That would be a loss of 11.9 percent over the 25-year period. But if the 10-year trend continues, the District will have 697,000 people in 2000, and its loss over the 25-year period will be 2.1 percent, the report showed.
Maryland had an estimated 4.1 million people in 1975 and will have 4.6 million in 2000 if the long-term trend continues and 4.35 million in 1980 and 5.4 million in 2000 if the short-term trend continues.
Virginia, which had about 5 million people in 1975, would grow to 6.4 million in 2000 under the 10-year trend and to 67 million in 2000 under the five-year trend.
The question of which trend will persist even another two years is politically important because if the 1970-1975 pattern prevails, four more congressional seats would shift than under the 1965-1975 pattern.
For instance, under the long-term trend, the losers (besides New York) would be Ohio, Pennsylvania and Illinois, whose House delegations would be cut by two seats each, and South Dakota, which would go from two seats to one.
Besides Florida and Maryland, the gainers under the long-term trend would be Texas, with two extra seats, and Arizona, California, Oregon, Tennessee and Utah, with one extra seat each.
If the 1970-1975 trend continues, Michigan and Missouri would be added to the list of lossers - each of them dropping one seat. Colorado, with one extra seat, would be added to the list of gainers, and Florida would pick up four seats instead of three the bureau said.
Under either trend Florida and Arizona are expected to make the largest population gains and the District to have the greatest loss.
In the next 25 years the nation is expected to grow 22 percent. Average annual growth rates under the 1965-1975 trend would be 4.1 percent for the Northeast, 5.3 percent for the North Central region, 10.3 percent for the South and 11.6 percent for the West. Under the 1970-1975 trend, the annual rates would be 2.9 percent for the Northeast, 3.7 percent for the North Central states, 12 percent for the South and 11.7 percent for the West.
John F. Long, chief of population projections at the Census Bureau, said the recent migration movements indicate that "the trend toward deconcentration is here to stay for some time. Whether it will be as strong as in the 1970-1975 period, we don't know.
"The South and West have grown because land and labor costs are cheaper and, with the interstate highway system, individuals and businesses have found they don't need to be near a major northern or eastern transportation center. The nonmetropolitan areas have grown because, with earlier retirement, more people are looking for extra space, recreation opportunities and good climate," he said.