The decennial census, which unfailingly fascinates demographers and economists but rarely alters the course of human events, offers enough promise to change the political landscape of America in 1980 that Democrats and Republicans are already beginning to tabulate winners and losers on the basis of reapportionment forecasts.

South Dakota, for example, will lose half its delegation in the House dropping from two to one if population estimates hold.

Republicans, who control both congressional seats, are beginning to talk about trying to send the odd-man-out to the governor's office.

In Texas, Rep. Bill Archer's overwhelmingly Republican House district in the western part of Houston (Harris County) has become so populous since he was first elected in 1970 that it almost could be made into two congressional districts of about 400,000 people each.

Archer's political advisers are wondering if some of those Republican voters can be put into the neighborign district of Democratic Rep. Barbara Jordan, whose constituency has been steadily declining since her election in 1972.

The Democrats, who control the assembly and the governor's mansion in New York State, at present at just beginning to analyze what the projected loss of two House seats means, and how they can counter it by drawing new congressional district lines.

Democrats also are wondering how a population explosion in Florida's Broward County, between Fort Lauderdale and Miami, and a similar boom in the Orlando area, west to Pinellas County, will benefit the party which controls the legislature in Tallahassee.

These political machinations and more are the result of provisional population estimates released by the Bureau of the Census that show dramatic population shifts across the country since the 1970 census.

The winners and losers will depend not only on the final population count in 1980 but on how state legislatures flex their muscles when it is time to draw the new congressional district maps for 1782. Thus fights for dominance in state capitols will take on added importance in the next few years, because if the past is any measure, the new lines will be decided partisan.

Already sounding a warning, David Cohen, president of the Washington based public interest citizens' lobby Common Cause, said: "Unless [reapportionment] reform moves succeed before 1981, politics at its rawest will prevail in most states when districts are redrawn."

Broadly speaking, big cities in the Northeast and in Northcentral states are losing population, while medium sized cities in the Sun Belt of the South and Southwest are gaining.

Using its own 1976 population estimates, the Census Bureau has forecast apportionment of 435 House scats following the 1980 census this way:

Florida will gain two seats, while one additional seat each will be apportioned to Texas, Tennessee, Oregon, Arizona and Utah.

New York will lose two seats and Pennsylvania, Illionois, Ohio, Michigan and South Dakota will lose one each.

While it is still too early to predict how the district lines will be redrawn by state legislatures - and in some cases by the courts - in 1981, some dramatic population shifts suggest obvious changes.

Since 1970, population in Portland Ore., has declined by 20,000, while suburban Washington County has had a population increase of 32.000. or 20 per cent. It is reasonable to predict that when Oregon picks up one more House seat, the line would be drawn to reflect the shift in population.

Similarly, losses of House seats in Pennsylvania and Michigan would result in redistricting that takes into account population declines of 125,000 in Philadelphia and 134,000 in Detroit.

The projected change in House representation is faithful to a trend that began in 1940, since which the North-east has lost 16 seats and the North-central states 10. During that period, the West picked up 27 House seats, led by California, which nearly doubled its representation from 23 to 43 seats.

Florida led the South in House gains, jumping from six to 15 seats from 1940 to 1970.

Gaining House seats as a result of the 1970 census were California, 5: Florida, 3, and Arizona, Colorado and Texas, 1 each.

This shift led to assumptions that Republicans would make major gains, but much of that optimism faded the next election year when Democrats picked up 11 new governorships and gained control of seven new state legislative chambers.

However, the same kind of optimism is now being voiced again by the Republicans, who view the decline in population of the Northeast's big urban centers as a liability to the Democrats, and the population gains in the medium-sized Sun Belt cities a propective plus for the GOP.

"In a broad sense, the shifting patterns of the population are advantageous to us.The growth areas are in areas of the country that was see as growth areas for us - energy-conscious states and job-oriented states. These are issues that we are confortable with, and they are in the areas that we do best in," Republican National Committee Chairman Bill Brock said in an interview.

Medium-sized Sun Belt cities, Brock said, "are exactly that category of community in which we have made enormous strides recently. We are encouraged by the [census] projections."

The GOP had a computer analysis of the population shifts made an projected gains similar to those of the Census Bureau, except that Florida would gain four House seats.

At Brock's prodding, the Republican Party has embarked on a program of rebuilding at the statehouse level, prompted by a belief that the success of any future national ticket will hinge on a grass-roots broadening of the party.

But the effort also is based on the realization that whoever controls the state legislatures and governorships in 1980 will control the redistricting.

Brock said the national committee's major expenditures at the local level will be in states with large congressional delegations and where the party feels it is within striking distance of taking atl east one legislative house and a governorship.

Republicans now control both legislative house and a governorhsip.

Republicans now control both legislative houses in only five states, and have only 12 of the 50 governors, a disadvantage that could be devastating when redistricting time arrives.

Not surprisingly, Democratic strategists also saw good fortune in the 1980 census estimates, saying that their traditional strength in the Souh will offset losses in urban Northeast and Northcentral states.

Scott Wolff of the Democratic National Committee's campaign services division said the DNC has not yet combined the Census Bureau's congressional apportionment estimates with county populations in the affected states, and therefore has little idea which districts would change.

"But the fact that most of the new seats are in the South and the losses are in the North would not work against us. We did pretty well in both regions the last time around," he said.

By constitutional mandate, the 1980 census results will go to the President by Jan. 1, 1981, and then to the House, which will apportion its 435 seats according to the population and direct the state legislatures to redraw district lines according to local population shifts in keeping with the one-man, one-vote concept.

The other major guideline imposed on the states is that congressional districts, which will be cast in time for the 1982 elections, must be contiguous.

Even so, the effects of redistricting can be pervasive, as both parties discovered in 1972. Only 13 seats were captured by either party without the influence of redistricting.

A congressional Quarterly survey showed that 17 of 24 seats that moved to the Republican side did so largely because of redistricting. The Democrats won eight of 14 new seats by way of redistricting and reapportionment.

Also, the combining of home districts of two incumbents occurred 28 times in 1971-72, usually to the disadvantage of the party out of power in the state capitol.

New York, with Republicans then in control of both houses of the legislature as well as the governorship, enacted a redistricting plan in which two Democratic districts were eliminated in New York City, a republican district was created in Long Island, and two upstate district with Democratic incumbents had Republicans added to them.

The practice of gerrymandering - redrawing congressional and state legislative district lines primarily on partisan considerations - has become a new target of Common Cause.

Based on anti-gerrymandering provisions in effect in Colorado, Hawaii and Montana, the Common Cause proposal would require single-member districts, create a nonpartisan reapportionment commission to draw new district lines, and provide for swift judicial review.

In most states. reapportionment reform would require a constitutional amendment.

"Reapportionment commissions can remove the conflict of interest inherent when legislators draw their own districts and can eliminate the abuses that have resulted from this system," Cohen said.