The opportunities that the 1980s hold for the Republican Party, and the perils they pose for the Democrats, are written in the numbers that the Census Bureau uses to describe what happened to America during the 1970s.

Those figures whow that during the last decade 30 congressional districts lost between 10 percent and 50 percent of their population. Geographically, these districts, the places Americans are gradually abandoning, stretch from the East Coast only to the west bank of the Mississippi River and only two of the 30 are south of the Ohio River. They are overwhelming northern and eastern and they have one other thing in common: every one is represented in Congress by a Democrat.

The Census Bureau lists another 30 congressional districts where population grew the most during the 1970s. These places, the winners in the regional competition for economic growth and political clout, have equaly distinctive characteristics.

Twenty-one of the fastest-growing districts are west of the Mississippi. Of the nine in the eastern half of the country, seven are in Florida, one in Georgia and one in Tennessee. They are overwhelmingly western and southern and the GOP already holds a clear edge -- 18 of the 30 districts are represented by Republicans.

The figures from the 1980 census confirmed what has long been known about growth patterns. America's population is shifting to the South and West. Now, in state capitals around the country, politicians are redrawing the lines of congressional and state legislative districts to reflect those shifts and equalize apportionment, as political power inevitably follows the path of the voters.

New York, the biggest loser, must give up five of its 39 seats in the House, as well as five of its 39 votes in the Electoral College. Florida will gain four seats.

Pennsylvania, Ohio and Illinois will each lose two seats, while Texas gains three and California picks up two. Losers of one seat each include states with strong liberal, labor union and Democratic Party traditions such as Massachusetts, Michigan and Missouri. Many of the gainers are clustered near the Rocky Mountains, states like Arizona, Colorado, Utah and Nevada, where political traditions and voter attitudes are far more conservative.

In all, 17 House seats (and electoral votes) will shift from the North and Midwest to the South and West as a result of redistricting before the 1982 congressional elections.

The potential for significant gains by the Republicans from this process are obvious. Richard Richards, the chairman of the Republican National Committee, believes the GOP will pick up a net of 10 to 12 seats as a result of redistricting which, coupled with what he predicts will be continued high popularity for President Reagan, could give the Republicans control of the House when the 98th Congress convenes in January, 1983.

"If you look at the demographics and at history, you can say the Democrats have a tough road ahead," said Kimball Brace, a Democratic political consultant who believes the GOP may gain more than 15 House seats as a result of redistricting.

Brace and others note another disturbing trend for the Demorats in the census figures: The movement of voters has not just been from the more Democratically inclined Northeast and industrial Midwest to growing Republican areas in the so-called Sun Belt states. Within states, it also has been out of the traditionally Democratic cities into the traditionally Republican suburbs.

Almost all the 30 big population losers among the nation's congressional districts are in the big cities of the Northeast and Midwest -- New York, Chicago, Philadelphia, Pittsburgh, Cleveland, Detroit, Baltimore and St. Louis among them. These districts lost population not just to the exploding Sun Belt, but to newer suburban areas in far out-counties that 10 years ago might have been mostly pastureland. Thus, while Chicago lost population and its older, close-in suburbs barely held their own during the 1970s, Du Page County, Ill., farther to the west, grew more than 30 percent.

Robert Teeter, a leading Republican pollster, predicts that the net effect of these population shifts from the cities to suburbs and from the North and Midwest to the Sun Belt will be "a significant decrease in the number of safe Democratic seats of recent years have not been in what was once known as the "Solid South," but in the urban centers that have lost the most population.

"If you increase the number of competitive seats you automatically increase the chances of Republican gains," he said.

Tennessee offers a graphic illustration of some of these trends. Between 1970 and 1980, the state's population went from 3.9 million to almost 4.6 million, a healthy growth rate of 17 percent that was enough to earn it an additional seat in Congress via the redistricting process.

Within Tennessee, the most spectacular growth took place in the 6th Congressional District, a huge chunk of land that includes suburban areas around both Memphis and Nashville. That's district's population grew by almost 40 percent during the last decade. Since 1972, it has been represented in Congress by Robin L. Beard, a Republican.

But in the midst of the general growth that Tennessee enjoyed during the 1970s, there was one exception. That occurred in the 8th Congressional District, which consists almost entirely of the city of Memphis. While the state's population was growing by 17 percent, the population of the 8th District was declining by 17 percent.

The 8th District is represented in Congress by Harold E. Ford, a Democrat and a black. Although Ford is likely to retain his seat, he symbolized many of the problems confronting the Democrats as the population shifts. Of the 10 congressional districts that lost the most population during the 1970s, seven are represented in Congress by black Democrats.

"Blacks have made the same choice as whites to raise their children in the suburbs," said Ann Lewis, the political director at the Democratic National Committee.

Census figures confirm a gradual dispersal of blacks to the suburbs. But black members of Congress are going to want to maintain their largely black constituencies and may be tempted to form what Brace describes as an unholy alliance with the Republicans in order to protect themselves.

Brace argues that the price of crafting overwhelmingly black and therefore safely Democratic congressional districts will be to turn over control of the growth areas of the country to the opposition. "If the Republicans can convince the Democrats to take safe, central-city districts, they are going to have free command in the suburbs," he said.

Despite these generally unfavorable population trends, the Democrats are not without their weapons and their hopes for the future as the redistricting process picks up steam in state capitals across the country.

Perhaps the Democrats' most important weapon is their control of a majority of the legislatures in the Sun Belt states that are gaining the congressional districts. In two of the largest, Florida and California, there are also Democratic governors, assuring the Democrats of control of the redistricting process.

Texas has a Republican governor but an overwhelmingly Democratic legislature. And even in the increasingly Republican Rock Mountain states, the Democrats control either the legislature or the governor's office, meaning they will a have a role in drawing the new districts in those states.

The Democrats' hopes for the future are most forcefully voices by Lewis, who speaks of a "demographic fallacy" in the population growth figures coming out of the Census Bureau. Lewis argues that not everyone who moves from the city to the suburbs or from the Northeast to the South or West is a Republican or certain to become a Republican once they are resettled.

"When you get on the airplane, they do not say, 'Coffee, tea or change of party registraiton form,'" she said, arguing that many of the people who are resettling in the growth areas are Democrats who will retain their party affiliation and voting habits.

Two other Democrats also say they see less reason for gloom than might be supposed from shifts in population.

Michael Barone, a co-author of the Almanac of American Politics and a vice president of Peter D. Hart Research Associates Inc., a Democratic polling firm, said one of the most interesting trends was "the migration that didn't take place" during the 1970s. The historical movement of blacks and poor whites from the South to the North came to a halt during the last decade and may be reversing itself, he said.

The likely result of this development, Barone believes, will be a moderation of southern politics. Black Democrats from places like Detroit and Chicago may be endangered by redistricting, but their seats in the Congressional Black Caucus could be taken during the 1980s by Democrats from places like Rocky Mount, N.C., he suggested.

Patrick H. Caddell, former president Carter's pollster, makes the same point, arguing that the South is likely to become "much more moderate" in its politics in the years ahead.

Both Caddell and Barone argue that it is far from certain that the future belongs to the Republicans in either the South or along the West Coast, where Democratic positions on environmental and social issues may be more attractive to voters than those of the Reagan administration. But to hold their own with the Republicans in these areas, Caddell said, Democrats from the North and East are going to have to chance their attitudes to conform with the new political party.

"The northern wing always treated the rest of the party with total disdain and inattention," he said. "The problem is that now that is where the votes are."