The just-published census figures for Fairfax County portend a significant reshaping of the political landscape in Northern Virginia's most populous jurisdiction.

Representatives of older supervisor districts that have been losing population are likely to find themselves in what could be a politician's nightmare -- representing entirely new neighborhoods with thousands of new names and faces.

Likewise, thousands of voters are likely to find themselves with new supervisors, even without an election.

As if that weren't enough, the older suburbs inside the Beltway and south of Alexandria are likely to find their political power slipping even further from their grasp -- toward the growing western areas of Fairfax.

The driving force behind these changes would be a realignment of the county's eight supervisor districts to make them roughly equal in population.

Preliminary 1980 population estimates for Fairfax, recently released by the U.S. Census Bureau, show major differences in the districts. Four districts -- Springfield, Centreville, Dranesville and Annandale -- have been growing, while the other four -- Lee, Mount Vernon, Mason and Providence -- have been losing population.

The county is required by state law to realign the districts at least every 10 years (after the decennial census) if there are major population imbalances. During a previous redistricting in 1976, the supervisors were able to get by with fairly minor adjustments that kept most districts relatively intact. But at the next redistricting, expected early next year, the supervisors will have to do far more than trim a bit here and add a bit there.

In fact, say some county observers, the new population firgures show imbalances so extensive that they can be corrected only by drastic remapping, or by creating a new district.

Unless the board carves out a ninth district -- which some supervisors say is a possibility -- some of the declining districts may have to stretch westward toward the new population centers.

For example, Lee and Mount Vernon have always been within the area bordered by the Beltway, the Potomac River and I-395 (Shirley Highway). But those two districts also have steadily lost population in the last five years.

Drawing new district lines to reflect those population changes probably would mean including a large part of Lee in Mount Vernon. Lee's geographical area then would be expanded, almost certainly making a historic jump across Shirley Highway into areas that have always been identified with Springfield and possibly Annandale.

And Lee's longtime supervisor, Democrat Joseph Alexander, could find himself somewhat in the shoes of a first-time candidate, trying to discover who his constituents are and what they want.

By contrast, the growing districts -- such as Springfield, which is 50 percent larger in population than Lee, Mount Vernon or Mason -- would have to be shorn of some big chunks of real estate.

But supervisors from those districts are not likely to give up any strongholds of support without a fight.

For instance, the Centreville supervisor, Democrat Martha V. Pennino, began her political career in Vienna and still lives there. Redistricting could require that she give up part of her home base. But that might be preferable to giving up a part of populous Reston, which traditionally has supported her more enthusiastically than her hometown.

Springfield, the most populous of the districts, with 94,163 people, probably will face the biggest geographical cuts. It's doubtful, however, that Republican Marie B. Travesky would want to lose Greenbriar on the north or some of the voting precincts in the east that helped her win a second term last November.

Extensive realignment of district lines will give more clout to residents of expanding western sections of the county -- areas where residents have been clamoring for more schools and other expensive services. The older sections of the county could find themselves increasingly isolated in battles of how to handle -- and pay for -- increasing growth in Fairfax.

The supervisors could avoid some of these dilemmas by creating a ninth district -- possibly a crescent-shaped area that could include parts of Springfield, Centreville and Dranesville, the three population leaders.

But that could create just as many political problems as realigning and retaining eight districts. With a ninth district, supervisors probably would want to eliminate the present at-large chairman's position to avoid tie votes.

If that happened the supervisors would choose a chairman from their own ranks, but it is doubtful that he or she would have the prestige and power of a chairman elected by all county voters. In addition, such a change would require approval by the General Assembly.

If history is any indicator, the supervisors are apt to draw lines that will minimize damage to their power bases. In 1976, they rejected all five staff proposals for redistricting and drew lines that disrupted their constituencies as little as possible.

But the population differences are so great this time that, however the supervisors wield their red pencils, it seems almost certain that politics in Fairfax County will face historic and -- for some residents and officeholders -- unsettling changes.