Fairfax County Democrats, shaken by the Republican sweep that gave the GOP control of the county Board of Supervisors for the first time since Reconstruction, said yesterday that their party's power is declining and remains vulnerable to continued defeats.

Republicans jubilantly declared that T. Farrell Egge's win in the bitterly contested race for the open Mount Vernon seat on the board, cemented the party's steadily growing political grip on the most populous locality in Virginia.

"Fairfax now will be recognized for what it has become -- more Republican than Democratic," said Warren E. Barry, a former state legislator who last year became the first Republican ever elected to clerk of Fairfax Circuit Court.

Democrats conceded yesterday that their long-held power base in the county has been eroded severely in recent years by strong Republican gains in both countywide elections and state legislative races.

Democrat Martha V. Pennino, whose position as vice chairman of the nine-member county board may be jeopardized by the new GOP majority, noted: "It seems like the Democrats are on the decline."

Leaders of both parties predicted the new GOP power, which will give the party control over appointments to dozens of county boards and commissions, could have a dramatic impact on political campaigns.

"There's been a perception that some Democratic incumbents were invincible," said Marie B. Travesky, a Springfield Republican who was defeated in a party primary last year in a bid for reelection to the county board. "This may give Republicans more courage to go after incumbents."

Some Democrats blamed the Republican gains on weaknesses in their party's campaigning and fund-raising efforts.

"The great equalizer to the Republican funds for Democratic candidates used to be the effective grass-roots organization of the Democratic Party," said state Sen. Joseph V. Gartlan, whose legislative district includes much of the Mount Vernon area. "Maybe this isn't the equalizer any more. . . . If that's the case, we've got to concentrate on improving our fund-raising, computerization and technical abilities."

The campaign for the Mount Vernon seat, left vacant by Democrat Sandra L. Duckworth, who unexpectedly resigned last summer to join her husband in a new job in Hawaii, was one of the most costly district campaigns in county history. Egge, 42, a lawyer, raised substantially more than his Democrat opponent, Gerald W. Hyland, or independent Gerald A. Fill.

While Hyland's loss could have been symptomatic of the Democrats' national problems, some Democrats said yesterday that the Fairfax loss could have been avoided.

Egge won with 46 percent of the vote -- only a 677-vote edge over Hyland. Fill received 10 percent of the vote, enough to change the outcome of the race.

"The Democrats ought to lynch themselves," charged Supervisor Joseph Alexander, the senior Democrat on the county board. He said the party opened itself to a strong assault when it allowed Duckworth to resign before the general election with a popular incumbent Republican president rather than wait until January and then force a special election for the Mount Vernon seat.

The Democrats' problems were compounded, according to Hyland, by Fill, who had been appointed to the school board by Duckworth, a Democrat, but who was forced to run as an independent because of his federal job.

"It hurt us," said Hyland, an attorney who has served on the county board of zoning appeals and human rights commission. "The bottom line is that Fill took more votes from me than he took from Egge."

Republicans and Democrats point to another important factor in the increasing popularity of Republican officeholders in the county.

"It's becoming a more conservative county," said Republican Supervisor Thomas M. Davis III. "It's a white, middle- and upper-middle-class county."

County supervisors said they expect no major upheavals on the board immediately after Egge is sworn into office at a Nov. 19 board meeting. The board traditionally makes its appointments in early January.

County Board Chairman John F. Herrity, a Republican who until now has lacked a working majority of GOP supervisors, has refused to discuss possible appointments the board may make, saying only "I have some ideas."

Although the board seldom casts votes along party lines, some Republicans are predicting the new GOP majority will make an already conservative board even more fiscally conservative.

Other Republicans counter that there will be little change. But Davis said the burden now will be on the GOP. "You've got to do something to show you're different," he said. "What else are you going to do besides say you'll cut taxes? That's a gut issue."