The 1980s have confounded Virginia's population forecasters.

Many of the state's leading cities, which were expected after the 1980 census to suffer a decline in population in this decade, are actually growing robustly. And while there was never any doubt that populous Northern Virginia would continue its postwar boom, analysts were caught off guard by the rate at which new residents have flocked to the suburbs south and west of Washington.

Virginia, which ranks 13th in population among the 50 states, is growing faster so far this decade than the nation as a whole, according to figures compiled by the Tayloe Murphy Institute at the University of Virginia. And 90 percent of the state's population boom is occurring in urban and suburban areas.

Virginia's overall population increased by an estimated 288,700 to 5,635,500 from 1980 to 1984, with Fairfax County and Virginia Beach accounting for 40 percent of that growth. The fast-growing "outer" suburban counties of Northern Virginia -- Prince William, Stafford, Loudoun and Fauquier -- made up another 12 percent of the state's population rise.

Northern Virginia is "where the jobs are . . . and that's where the people are moving," said Fairfax County Supervisor Thomas M. Davis III (R-Mason). "It's been going on for 30 years and it's accelerating."

Northern Virginia politicians and bureaucrats, once a lonely minority in a state dominated by rural interests, see further confirmation in the new demographic figures that their day in power is at hand. They reckon that the region's population explosion signals a continued power shift that will favor a coalition of Northern Virginia and Tidewater legislators in the state's General Assembly.

"A decade ago the Virginia General Assembly was dominated by rural interests," said Sen. Wiley F. Mitchell (R-Fairfax). "Things changed in the 1980 census so that there was at least a standoff between the rural, agrarian interests and the more urban interests.

"The 1990 census is very likely to shift the balance of power from a fairly even division now to an overwhelmingly urban orientation," Mitchell said.

Fairfax County Board Vice Chairman Martha V. Pennino (D-Centreville) agreed with Mitchell's analysis, and said the population swings will have broad policy implications in the state legislature, sharpening the focus on transportation and education issues in particular.

"For years the people in urban areas complained . . . that the rural boys were giving it to us," said Pennino. "Now it will change. It's time that the urban area shortages in roads and public education be addressed."

The new clout of the Fairfax and Tidewater coalition was seen at last winter's session of the state legislature, when lawmakers from the two areas combined to push through a new formula governing the distribution of road funds that benefited urban and suburban areas at the expense of rural districts.

More recently, the two areas combined this summer to pressure a state panel that had urged a relaxation of state building maintenance codes to back down and restore the tougher guidelines favored by populous regions.

Despite the power attending the new numbers, however, some legislators see political pitfalls as one byproduct of rapid population growth.

"Growth also means instability," said state Del. Vivian E. Watts (D-Fairfax). "New people don't know who's currently on board and who's doing good work. It can be a very volatile situation that doesn't necessarily translate into power and results in the General Assembly."

She said the comparable seniority of Tidewater legislators, most of them Democrats who are returned to office without significant challenge, is not matched in the legislature by Northern Virginia representatives, who frequently face tough reelection races.

"Tidewater is still the most powerful area of the state because of the seniority of its members," she said. "The legislative game is seniority. It's not enough to have sheer numbers."