For Northern Virginia Republicans, 1987 was one long horror movie. Call it a political sequel to "Invasion of the Body Snatchers."

Last January the GOP controlled five of nine seats on the Fairfax County Board of Supervisors, a majority; now the party has a minority of two. The sole Republican on the five-member Arlington County Board just retired, and a string of highly touted GOP challengers for the state legislature went down to defeat.

The party also lost a high-profile bid to unseat the Prince William County commonwealth's attorney, and held on to only one seat on both the Prince William and Loudoun county boards. In Alexandria, only one Republican incumbent is in the race for the seven spots on the City Council.

Not since the early 1970s, when Virginia Democrats held a historic one-party stranglehold on state and local government, have things gone so badly for the Northern Virginia GOP. Over the last 15 years the Republican Party had been a growth industry in the suburbs, chipping away at Democratic control of the State Capitol and of county courthouses and city halls across the region.

But last year the Republican upsurge stopped -- at least temporarily -- and activists in both parties have attempted to determine why. In interviews they pointed to several factors:Voters, infuriated by traffic-choked highways and what they saw as uncontrolled growth, turned against Republicans, who were viewed as friendly to the real estate development industry. Northern Virginia Democrats have effectively tapped local businesses for campaign contributions, neutralizing a clear monetary edge that Republicans once enjoyed. State Sen.-Elect Emilie Miller, a Fairfax Democrat, raised more money than the incumbent Republican she defeated. And despite considerable opposition from developers, new Fairfax County Board Chairman Audrey Moore collected more than $300,000. Fighting within the GOP, much of it pitting conservatives against moderates, has weakened the party structure and hurt its candidates. Former Fairfax supervisor Nancy K. Falck was defeated in part because a fellow Republican ran as an independent in the general election, enabling Democrat Lilla Richards to win in a three-way split. Republican candidates with ties to Capitol Hill or national GOP organizations were unable to transfer that strength to local politics. Six Republicans with recent Washington experience ran for the General Assembly in November; all lost. There were no national seats up for election in the state this fall, and voters draw a clear distinction between national and local elections, sometimes ignoring local balloting.

The good news for Republicans is that the damage probably is not permanent. With two veteran congressmen, 8th District Rep. Stan Parris and 10th District Rep. Frank R. Wolf, standing for reelection in 1988, the Northern Virginia GOP will have a strong foundation this year. And according to Larry J. Sabato, a political scientist at the University of Virginia, history does not always predict Northern Virginia's future.

"One election year is disconnected from the next," Sabato said. "For each race you really have different issues, different personalities and a different electorate.

"A substantial portion of Northern Virginia residents don't turn out for local elections because they don't identify with the area. And in every election the voters choose from different candidates, with different strengths and weaknesses."

Still, the only encouragement Republicans find in 1987 is that it's over. And in political postmortems for the year, the GOP's low point is Moore's landslide defeat of Republican John F. Herrity to head the Fairfax County Board.

"I don't think Mr. Herrity would have gotten elected if he had had $5 million to spend," said Fairfax Republican Chairman James D. Swinson Sr. "The issue here was transportation and growth, and Mrs. Moore sold it. People came out to vote for Mrs. Moore, and then they voted for the Democrat right next to her."

In the wake of Moore's tidal wave, politicians across Northern Virginia are making "controlled growth" a catch phrase. State Sen. Wiley F. Mitchell, an Alexandria Republican, said he expects antidevelopment sentiment will influence Alexandria City Council races this spring. "The voters had a very clear message" in Fairfax, Mitchell said. "They were concerned about growth and they wanted to turn out incumbents."

But other, less noticed factors also contributed to the Republican losses. One is a longstanding dispute between traditional party activists and conservative newcomers, many of them from the Christian right, that has hurt the GOP both in Fairfax and Arlington.

The most obvious example of this came in Falck's race for the Fairfax County Board. Falck defeated Lloyd Thoburn, the administrator of a Fairfax Christian school, in a party primary. But in the general election Thoburn's father Robert, a former member of the House of Delegates, entered the race. He siphoned off enough votes to cost Falck the election.

And while GOP differences fester, the region's normally combative Democrats have made peace. "The Republicans have all sorts of problems," said 10th District Democratic Chairman Daniel Alcorn. "But on the Democratic side I would be hard pressed to tell you of any division. It's a unique phenomenon."

"We have been through some very serious divisiveness," said Arlington GOP Chairman Scott McGeary. "And every time Fairfax has a county chairman's race it's a world war. These contests have carried over for years."

Republicans have also lost an edge they traditionally held in campaign fund raising. Sabato found that in the contested races for the General Assembly statewide, Democrats outspent Republicans by a ratio of almost 3 to 2. And in Northern Virginia the Democrats' incumbents and challengers were generally free of serious financial woes.

The Democrats have actively courted donors, particularly business executives, in recent months. Donald Beyer, a car dealer who gave more than $50,000 to the campaign of Democratic Gov. Gerald L. Baliles, has formed the Fairfax Democratic Business Forum, which draws several hundred people to its meetings. And the election of two consecutive Democratic governors has given many donors who once supported GOP candidates a reason to hedge their bets.

"A lot of business people don't give out of some great philosophical belief. They give to be with a winner," said Republican Thomas M. Davis III, one of the two Republicans left on the Fairfax County Board. "Right now the Democrats have the financial advantage."

Fairfax Democratic Chairman Harris N. Miller said, "I thought Audrey Moore could raise $300,000, but nobody else thought she could. We've been able to show contributors that the Democratic Party is not some fringe or splinter group."

Republican candidates found out the hard way last year that experience in national politics does not form the basis of a winning campaign. A half-dozen GOP activists who had previously worked for Congress or the executive branch, including Fairfax state Senate contender Bobbie N. Kilberg, became proof of an old adage: all politics is local.

"The voters ain't dumb, and if you don't have a broad base of community activities you aren't going anywhere," said Del. Warren G. Stambaugh, an Arlington Democrat. "People here say, 'He works on Capitol Hill? So what? I know 25 people who work on Capitol Hill.' "

Davis of Fairfax County said, "I spent two years working in the White House and I never put it in my campaign literature. It's not relevant to local elections. I do put in my literature that I was president of my civic association and a member of the Rotary Club."

Both Republican and Democratic leaders said that the key to both parties' long-term success in local races is recruiting candidates with a strong background of community service. They said that involvement in neighborhood causes and civic affairs is usually more important than partisanship or philosophy in winning elections.

"Political trends wax and they wane," said Mitchell of Alexandria. "But the voters of Northern Virginia are very sophisticated. They tend to stay in the mainstream. And neither party has enough strength to elect a clearly inferior candidate over a candidate who is clearly superior."