Immigrants made up more than one-third of the nation's growth in the 1980s, driving population change more than at any time since the early part of this century.

International newcomers accelerated the extraordinary growth in the West: If California had attracted no immigrants, it would have gained two new seats in the U.S. House of Representatives instead of seven.

At the same time, newcomers from abroad replaced a rapidly departing native-born populace in a band of states stretching across the Midwest and Northeast, stemming or reversing what could have been devastating population losses.

If it were not for nearly a million immigrants settling in New York, for example, the state would have lost population; instead it registered about 3 percent growth. The same is true in Illinois, which drew more than 400,000 immigrants but grew by fewer than 40,000 residents.

And in the Washington suburbs, the arrival of Hispanic and Asian immigrants contributed to double-digit growth during the 1980s.

Federal estimates analyzed by the Urban Institute put immigration in the 1980s at 104,577 for Virginia and 93,969 for Maryland, ranking the states 11th and 12th in the nation, respectively. The District drew 25,583 immigrants, ranking 31st.

"Without immigration, you'd have a much different picture of population change," said Jeffrey Passel, a demographer at the Urban Institute.

While a detailed picture of immigration will not be available from the 1990 census for several months, existing estimates and state population totals issued last week by the Census Bureau confirm that immigration has become central in deciding the nation's gainers and losers.

Population figures are used to determine the shares of about $28 billion annually in federal funding and to determine political representation. (The Census Bureau makes no distinction between legal and undocumented immigrants in counting population for political reapportionment.)

California, Texas and Florida, which ranked first, third and fourth in 1980s immigration, together won 14 new congressional seats in the 1990 apportionment. New York, which ranked second in immigration, lost three House seats, a product of the departure of more than a million of its residents.

Even in states such as Indiana -- more than 90 percent white and native born -- immigration became increasingly important in the 1980s. The state's 36,000 immigrants provided the bulk of its population growth.

In the northwestern corner of Indiana, where steel mills and blast furnaces line the lower tip of Lake Michigan, a growing minority and immigrant community seems estranged from the rest of the state.

"They say there's Indiana, and then there's 'The Region,' " said Carlos Alvarez, president of the area's Hispanic Chamber of Commerce. "The Region," he said, refers to the northwestern industrial pocket that "was always a melting pot."

But never before has the stream of immigrants to this corner mattered so much in maintaining the state's growth.

The Census Bureau estimates that 6 million legal and 2 million undocumented immigrants came to the United States during the 1980s, when the total population grew by about 22 million.

That level of immigration is second only to the 8.8 million foreigners who arrived between 1900 and 1910.

"We're in a second great wave now that's been steadily building," said Passel.

The first wave of immigration, which began in the 1840s and continued until World War I, contributed a greater percentage of growth in what was then a much smaller nation.

Still, the current proportion of increase caused by foreign migrants is greater than any since the decade between 1910 and 1920, when immigration accounted for 40 percent of the nation's population growth.

At the same time, demographers are quick to point out that the nation has attracted large numbers of foreigners for virtually all of its history and that, as a proportion of the total population, foreign-born residents still account for only about 7 percent.

"We're still a country of mass migration, but not like it was early this century," said Charles Keely, a professor of international migration at Georgetown University.

Many experts believe that the next decade will bring an equal or greater stream of immigrants, the result of liberalized laws and the continuing economic and political factors that push foreigners to American shores.

"There's every indication it will be at least as much of a factor in growth as it was in the 1980s," said Frank D. Bean, a professor of sociology at the University of Texas. "The numbers will be higher."

The uneven distribution of immigrants across the country -- seven states received 75 percent of total immigration -- is largely due to two factors that dominate when immigrants are choosing where to set- tle.

According to Bean and others, newcomers have historically selected communities with job opportunities or where friends or family have settled. In many cases, those two overlap when friends or relatives smooth the way for an immigrant to find a job.

It is this social network, for example, that explains the large numbers of Hmongs, a tribal group from the mountains of Laos, drawn to Minneapolis and St. Paul. While these cities are known more for their historical attraction to European immigrants than as a home for Asian newcomers, the stream of Hmongs began with church-sponsored refugee programs, then gained momentum as more and more family members followed.

At the same time, rapid growth in some states had less to do with immigration. Florida's 33 percent growth in the 1980s was primarily due to movement from other states. Florida attracted 2.8 million migrants, but 2.3 million of those were from within the United States.

By comparison, California drew 3.2 million migrants, 2.3 million of whom were from foreign countries.

If it were not for that level of immigration and the distribution of those immigrants, said Passel, "we'd be talking about Florida as the principal growth center, not California."