The day after slow-growth candidate Scott K. York won the top elected job in quickly growing Loudoun County, he laid down a challenge for himself and other candidates who swept into office on a pledge to slow the build-up of the outer suburbs.

"If I move this county forward to have it end up looking like Fairfax County, the citizens ought to take me after four years and string me up and shoot me," said York, who beat an incumbent to become chairman of the Loudoun County Board of Supervisors. "It would be a shame at the end of the day if we paved over Loudoun County."

York and other winning candidates in Loudoun and Prince William counties, who spent months promising to slow development in Washington's outer suburbs, yesterday confronted the daunting task of transforming their campaign slogans into action.

Several said they realized that winning Tuesday's election may have been the easy part.

And some business leaders warned that slow-growth measures promised by the victors would dampen the booming local economy.

York (R) won Tuesday as part of an eight-person slate of growth-control advocates that will take control of Loudoun County's nine-member board of supervisors in January. In Prince William, Republican novice Sean T. Connaughton defeated veteran county board Chairman Kathleen K. Seefeldt (D); Connaughton had said she was too tolerant of rapid development that has nearly doubled Prince William's population in the past 15 years.

Yesterday, York announced he would form a transition team to work on plans to limit home building and preserve more of Loudoun's open land. In Prince William, Connaughton promised to try to tame growth by pressing for incentives, such as tax breaks, for developers who agree not to build houses on their land.

Some business leaders said candidates may have oversold the slow-growth issue to voters, who may be surprised at how little can be changed.

"I don't have a clue what those people think they're going to do to suddenly stop growth or slow growth," said John T. "Til" Hazel Jr., a Northern Virginia developer for a quarter-century. "They will not be able to stop growth tomorrow, which is what they've implied to the voters."

One obstacle that slow-growth activists face is that officials in Prince William and Loudoun counties have previously approved tens of thousands of homes that haven't yet been built. Most proposals for slowing development wouldn't affect those approvals and are aimed at changing longer-range construction patterns. In addition, lawmakers in Richmond have been reluctant to give localities some of the authority they want to restrict construction.

Tuesday's election reminded some Northern Virginians of the mid-1980s in Fairfax, when voter reaction to rampant development catapulted slow-growth advocates into office. Their anti-development policies contributed to a recession a few years later, said Earle Williams, a statewide business leader and former technology executive. That could happen again, he warned.

"It appears to me that Loudoun is going to try to dig their moat around the county and fill it with water and alligators and pull up the drawbridge," Williams said. "It doesn't work."

Some voters said yesterday that they found growth such a compelling issue that they had turned against veteran leaders they now blame for encouraging sprawl.

Angelo Cardova, 75, of Woodbridge, a Seefeldt supporter in the past, voted for Connaughton this year because she said the current chairman hadn't done enough to slow growth.

"In the past few years, she was very much in favor of growth," Cardova said.

Tom Saavedra, 45, of Middleburg, said development has rolled through Loudoun like a "tidal wave." Now houses are being built in a field next to his property.

"People will put up with the growth until it hits close to them," said Saavedra, who voted for slow-growth candidates. "Then it flips the reset button and it becomes too much, too fast."

Even in heavily developed Fairfax County, some voters said they voted to oust incumbent Republican Robert B. Dix Jr. (Hunter Mill) because of concerns about traffic and vanishing open space in the Dulles corridor. Democrat Catherine M. Hudgins, who defeated Dix, said yesterday that she would try to find ways to plan better.

Members of the winning slow-growth slate in Loudoun have outlined specific measures to limit development. They want to rewrite county plans to reduce the number of houses allowed. They also want the authority to halt residential development in areas with inadequate roads and not enough classrooms, or when the county cannot afford new schools and other facilities for more residents. But to get some of those powers, they would need approval in the General Assembly, which has been unwilling to give it.

Loudoun's slow-growth winners said they also would seek to charge developers fees for every house they build and increase the amount they are expected to contribute in exchange for rezonings.

In Prince William, Connaughton said the county needs to find a way to reduce the number of homes that were approved for development years ago. He proposes offering incentives to developers for reducing the units per acre or switching from residential to commercial development.

Slow-growth advocates in both Prince William and Loudoun said they need to control growth but also promote tax-generating commercial development. Commercial development--favored by many Prince William officials as a way to offset the county's high tax rate--can also create demand for houses.

"That'll be the challenge--to be able to maintain that balance," said William D. Bogard (I), who was elected to represent Loudoun's Sugarland Run District.

"Loudoun is going to grow no matter what," said Charles A. Harris (D), who was elected in Loudoun's Broad Run District. "We just need to make it more reasonable, more planned, more phased, more thought out."

Staff writers Dana Hedgpeth, Michael D. Shear and Josh White contributed to this report.