The mounting anxiety of Northern Virginia voters over rapid development erupted in local elections yesterday, most dramatically in Loudoun--the nation's third-fastest-growing county--where a slate of eight slow-growth candidates for Board of Supervisors carried every contested race.

Similar concerns ambushed the reelection bid of Prince William Board of County Supervisors Chairman Kathleen K. Seefeldt (D), who was deposed by challenger Sean T. Connaughton (R) despite Seefeldt's efforts in recent weeks to recast herself as an opponent of growth.

In Fairfax County, Democrats bucked the statewide Republican tide, widening their majority on the county board by ousting two-time Supervisor Robert B. Dix Jr. (R), a vocal advocate for the business community and ambitious road construction. In besting Dix, newcomer Catherine M. Hudgins accomplished her goal of making the contest a referendum on his stance on sprawl.

But it was in the outer counties, Prince William and especially Loudoun, where candidates were most successful in tapping popular frustration over roads increasingly glutted with traffic, schools spilling into makeshift classrooms and the relentless harvest of new subdivisions.

Loudoun's population growth, more than 67 percent this decade, has accelerated to the point where about 1,000 new residents arrive every month. Prince William has recorded a growth rate of 21 percent this decade, so outstripping local services that county students are taught in about 150 classroom trailers.

"People are just very upset with the way the county has grown," Connaughton said. "The county has not done a good job managing growth. I think it's clear from the school issue, the transportation issue."

Sprawl was central to all eight contested races in Loudoun, most visibly the contest for board chairman. Scott K. York, a Republican slow-growth advocate, thrashed both his independent opponents, winning nearly two-thirds of the vote. Dale Polen Myers, the board chairman York had upset in the Republican primary after tarring her as an ally of developers, failed in her effort to revive her political fortunes as an independent. She finished third behind James G. Kelly, who ran on a slow-growth platform and raised less than $1,000 for his campaign.

York and other victors in Loudoun benefited during the campaign from mailings by Voters to Stop Sprawl, which warned that this election would be the last chance to save the county from developers. This helped some slow-growth candidates overcome their more meager campaign accounts.

The rising anxiety over development has been evident in Northern Virginia for more than a year, prompting some candidates, such as Myers and Seefeldt, to try repositioning themselves as advocates of "managed growth."

That proved to be a tough sell for Seefeldt, an eight-year incumbent as chairman whose voting record and hefty donations from developers were pummeled by Connaughton during the Prince William campaign. A Washington Post analysis had shown that she voted to approve 92 percent of the housing development requests before her. Seefeldt had said supervisors have little choice but to back projects that comply with the county's master plan.

The dean of the county board and a prominent figure in regional transportation planning, Seefeldt had chafed at suggestions that she was insensitive to the stresses caused by rapid development. But her message that the county was prospering because of economic development--such as the decision by America Online Inc.'s choice of Prince William for a new $520 million data center--proved unpersuasive. Nor did her hefty campaign treasury preclude her narrow defeat.

Fairfax was spared similar upheavals yesterday, but Dix's defeat was stunning all the same. Republicans had hoped to erase their slender deficit on the county board but instead saw the Democratic lead expand to 7-3, leaving Hanley and her fellow Democrats in clear command as the supervisors prepare this month to select a new county executive.

And as the board sets out to craft a new agenda on transportation and education policies, Dix's defeat will amplify the call by slow-growth advocates for a more measured development plan.

"I ran on the issue of the quality of life in the Hunter Mill District," said Hudgins, a former aide to Democratic Board Chairman Katherine K. Hanley. "We're going to be looking at this issue as a community, how we manage growth and give balance to growth and the quality-of-life issues."

Slow-growth advocates, however, did not record a clean sweep across Northern Virginia. Prince William County Supervisor Edgar S. Wilbourn III (R-Gainesville) held off a determined challenge by Democrat Gary C. Friedman, who had attacked the incumbent for his links to the building industry and opposition to the county's comprehensive slow-growth plan.

And in Fairfax, Supervisor Stuart Mendelsohn (R-Dranesville) won reelection easily despite sharp criticism from Democrats over his vote approving a developer's plan to plow under the Evans Farm property and erect town houses.

Still, the gains by slow-growth forces were enough for victory to be claimed by Stewart Schwartz, a spokesman for the Coalition for Smarter Growth. "I consider this a vindication. We have long said growth is the issue in Northern Virginia, particularly at the local level, and this confirms that," he said.

But Edward H. Bersoff, former chairman of the Fairfax County Chamber of Commerce, said he was disappointed by the gains made by slow-growth candidates in Northern Virginia, saying people who blamed increased housing developments for much of the traffic and school overcrowding held a narrow view.

"I'm not heartened by slow-growth candidates gaining the position of power, but all you have to do is work harder to argue that having balanced growth--not slowing down growth--is the right thing."

CAPTION: Virginia Gov. James Gilmore yells, "Free at last!" to the crowd at the GOP party at the Marriott Richmond Hotel.