Among the big city members of Congress of the Northeast and Midwest, it is a deadly game of musical chairs: when they all sit down, some will have lost their seats.

In the South and West, lands of sudden plenty, it is a birghday party with more than enough toys for everyone to argue over.

The restless movement of the American people over the last decade, now being measured and recorded by the U.S. Census Bureau, will result next year in a massive redrawing of the nation's political boundaries. The losers are already well known: big cities, the Northeast and Midwest, and urban blacks.

It is the decennial ritual of cutthroat politics in which Democrats can stick it to Republicans, conservatives to liberals, whites to blacks, incumbents to challengers and loyalists to mavericks.

Reapportionment is the time when some politicians get even and others get mad.

"It's going to be a real bloodbath in the major cities like Chicago and Detroit," said Sandy Gonzalez, an aide to Rep. Cardiss Collins (D-Ill.). Collins' Chicago district has lost about 13 percent of its population in the 1970s, and the city is likely to forfeit one of its seven house seats.

On the broadest level, the decade has seen a flow of population from north to south and from east to west which will sharply tilt the balance of power in Congress and in the Electoral College after 1982.

New York, for instance, is likely to lose four of its 39 congressional seats, while Florida gains three. Illinois, Ohio and Pennsylvania will probably drop two each, while California and Texas pick up two apiece. With 45 seats, California would hold more than 10 percent of the House of Representatives.

In all, the Census Bureau is predicting that 14 seats will shift as a result of the new head count, meaning a 32-vote Electoral College advantage for the more conservative Sunbelt -- a sharp increase from its current four-vote margin over the Frostbelt.

As important as the interstate shift is the massive movement of Americans within states -- mostly out of cities and into suburbs and small towns. The boundaries of virtually every congressional and state legislative district, as well as city and county council lines, will be redrawn in 1981 and 1982 -- an orgy of action for power brokers large and small.

The numbers will have broad effects: social programs and labor legislation may have tougher sledding through Congress and a liberal devoted to those causes may have a harder time getting elected president.

And the numbers will have local effects: Arlington and Alexandria could lose a state legislative seat to the outer suburbs. Massachusetts Democrats may try to gerrymander Republican Margaret Heckler out of her congressional seat. Steel, coal and auto companies will lose clout to the oil and aerospace industries. Hispanics may gain another representative if all the illegal aliens in Los Angeles are counted.

The dealing has already begun.

"I can see the jockeying," said Rep. Robert Garcia (D-N.Y.), whose Bronx district is one of the greatest population losers. "Everyone's looking to protect the seats they have."

In what is certainly political, if not poetic justice, the fate of members of Congress lies in the hands of state legislators who must redraw the lines.

"If past performance is repeated, David Cohen, president of Common Cause, warns, this redistricting process will include many closed-door conferences and confrontations, midnight flights from Washington to state capitals, and virtually incessant telephone calls from anxious congressional incumbents to state legislators.

"And in the end, these legislators, armed with the latest in sophisticated computer technology and voting data, will again twist and bend election districts to serve partisan and personal interests," he said.

Common Cause is supporting anti-gerrymandering bills in Congress and in state legislatures. Districts would have to be compact, follow existing county lines and vary in population by no more than 2 percent. Two congressional bills would require that independent commissions rather than state legislators draw the lines -- a practice now followed by Montana and Hawaii.

But the Democrats who control Congress and most of the legislatures have little incentive to give up the powerful tool of reapportionment.

Bill Brock, Republican National Committee chairman, is a former congressman who ran for office four times, each time in a different district, thanks to Democratic gerrymandering in the Tennessee Legislature.

Nationwide, after the 1970 census, Brock said, "The Republicans got pretty badly kicked. We've consistently gotten 10 percent more popular votes than we have seats, both at the state and congressional levels. That's what a gerrymander does. You lose the close ones because the Democrats draw the close districts to be marginally Democratic."

Now the Republicans mean to get even.

"State elections are our top priority," Brock said.

The RNC and its political action comittee, GOPAC, will have spent $8 million overall in 1978, 1979 and 1980 to elect the state legislators who will draw the magic lines next year.

"If we're going to be a majority party, we've got to win the reapportionment fight," Brock said.

The odds are against him, although the momentum may be swinging in the GOP's direction. Democrats control 31 governorships and 67 of the nation's 98 partisan legislative bodies (Nebraska's unicameral legislature is nonpartisan.) But Republicans scored impressive gains in 1978, and the margins in many state legislatures are very narrow.

With 96 additional seats won in the right places -- out of 5,917 state legislative races this fall -- Republicans could win control of 20 more legislative bodies. It would take only one more seat in the Illinois and Washington houses, for instance; two seats each in the Ohio Senate and the Minnesota House, and three seats apiece in the Delaware and Illinois senates and the Maine House.

"If I were a Democrat, I'd be worried," Brock said, boasting that the GOP has spent far more money and is way ahead in grass-roots organizing.

Over at the Democratic National Committee, which has devoted most of its comparatively meager resources to the presidential race, Deputy Director Robert Neuman says, "Am I worried? Sure, I'd be crazy not to be. But I'm not panic stricken. We're not going to be caught asleep.

"We can't match them dollar for dollar -- no way. And we don't have their sophisticated direct-mail operation. But they're playing catch-up. They don't have the manpower we do. Their plan is fine on paper, but I wouldn't put my life savings on it. We're not as afraid of Republicans as we are of our own internal split."

Regardless of the outcome of this fall's elections, the census seems bound to help the Republicans. Of the 106 congressional districts that have lost population since 1970, virtually all are in urban Democratic areas. For instance, Garcia's south Bronx district lost 39 percent of its residents; Charles Diggs' Detroit district, 28 percent; Louis Stokes' Cleveland district, 25 percent; Parren Mitchell's Baltimore district, 14 percent.

The result, according to political analyst Richard Scammon, is "a significant decline in the traditional power of the big Democratic machines and a waning national constituency for major urban programs."

The big population gainers are staunchly Republican areas: Clare Burgener's suburban San Diego district, 64 percent growth; Bill Archer's Houston district, 63 percent; Richard Kelley's northern Florida district, 58 percent; Kenneth Robinson's northcentral Virginia district, 23 percent.

However, some analysts point out that blue-collar workers who retirer to Florida on union pensions are still voting Democratic. "In many of the population shifts of the postwar era, people have tended to take their politics with them," says pollster Burns Roper. "We need more data on exactly who has moved south and west since the 1970 census."

Many big-city members of Congress are counting on seniority and political clout to protect them. No one in Massachusetts, for instance, is talking about cutting out Majority Leader Thomas P. (Tip) O'Neill Jr.'s seat, although his district has lost population.

Rep. Dan Rostenkowski (D.-ILL.), whose Chicago district has lost 18 percent of its population, says, "I'm not going to get excited about it. We can sit down and decide what we want to do in an amiable manner."

Of the 20 districts that have lost the most population, nine are represented by blacks. Reapportionment, says Collins, head of the Congressional Black Caucus, "is going to wreak havoc on blacks who represent big cities, just at the time when we've been trying to get all the political power we can."

But Neuman of the Democratic National Committee says, "I don't think anyone would have the chutzpah to disenfranchise the blacks."

Whatever the outcome of state and congressional races, presidential politics will be profoundly changed. The Frostbelt's expected loss of 36 electoral votes between 1960 and 1984 is as if Pennsylvania and Rhode Island suddenly had fallen into the Atlantic, political analyst Ben Wattenberg suggested.

Wattenberg speculates that 1980 for Edward M. Kennedy is "now or never," since the conventional wisdom of the last decade, that a candidate could win the presidency with only the votes of California plus the Northeast and Midwest quadrangle, will be far from true in 1984.

This census and the subsequent reapportionment, Wattenberg suggests, creates nothing short of "a new political terrain."