The defeat of two progrowth Republicans on the Loudoun County Board of Supervisors, one in a startling upset by his former wife, signals the beginning of a cooperative new era in which county officials will work in closer harmony to hold developers to tough standards, political and business leaders in the county said yesterday.

From the densely clustered town houses in eastern Loudoun's Sterling District to the rolling pastures in Catoctin 20 miles west, Tuesday's elections brought political disaster to supervisors Andrew R. Bird III and Frank I. Lambert, two of the board's most outspoken advocates of economic development and protection of private property rights.

Despite this, leading Democrats and land developers agreed that a dramatic renunciation of growth similar to what evidently took place in Fairfax County is not in the cards for Loudoun.

In the first place, enthusiastic advocates of growth in the mold of defeated Fairfax Board Chairman John F. Herrity have never held a majority on Loudoun's eight-member board. Furthermore, most county political observers said yesterday, even Loudoun's slow-growth advocates realize that much of the county's growth is inevitable.

"I don't see the board being against growth," said Board Chairman Betty W. Tatum, a Democrat unopposed for reelection to a second full term from eastern Loudoun's Guilford District. "I see the new board being very concerned about channeling that growth, making sure that the impact on the county would be paid for as much as possible by the developer."

Bird and Lambert are part of a vocal conservative minority on the current board and their combative stances often made consensus impossible, Tatum said.

Tuesday's most stunning upset was in Sterling, where independent Alice G. Bird scored a victory over two-term Republican Andrew Bird, her husband of 15 years until their divorce last year.

Alice Bird's campaign initially prompted snickers from many in Loudoun's political establishment, most of whom initially did not take her candidacy seriously and seemed to delight in the prospect of the dirty laundry of a failed marriage being hung out for public view.

The race produced no revelations, but Alice Bird's description of her ex-husband as politically aloof and too permissive toward growth struck a resonant chord with voters, who ushered her into office with 54 percent of the vote.

Likewise in Catoctin, first-termer Lambert dismissed Democratic civic activist Betsey J.S. Brown as "a great organizer of busy ladies." Yet voters -- apparently fearful that the rapid growth that has already transformed eastern Loudoun near Dulles International Airport is heading west -- endorsed her slow-growth message with 60 percent of the vote.

In Sterling and Catoctin, challengers were successful because they were able to paint the incumbents as incapable of balancing the county's inevitable growth against residents' fears that excessive development has worsened traffic jams and threatens Loudoun's scenic beauty and historic character, many political observers said.

"There's definitely a reaction to a development-at-any-cost attitude," said Robert E. Buchanan, a leading Loudoun developer, adding that county officials have "always been tempered in their stance toward development . . . . If you're looking for someone to say, 'The sky is falling,' that's not me."

Tuesday's balloting, in which Democrats gained two seats from the GOP, means that the board taking office in January will have six Democrats, one Republican and one independent.

In other supervisor races:Five-term Republican incumbent James F. Brownell, who ran in the Blue Ridge District as an independent this year, demonstrated anew an ability to pull political victory out of the most difficult circumstances by winning 42 percent of the vote against 41 percent for independent Ben F. Fordney, who was endorsed by the Democratic Party, and 17 percent for Republican Larry Johnson. In eastern Loudoun's Broad Run District, first-term incumbent Steve W. Stockman, a former independent running this time as a Republican, beat independent Gregory Rex Marquis with 65 percent of the vote. Democrat Thomas S. Dodson won a third term in the Middleburg area Mercer District over Republican G. Lawrence Moison with 69 percent of the vote. In the Dulles District, first-term Democrat Ann B. Kavanagh was reelected with 45.8 percent of the vote -- a nine-vote margin over Republican Benjamin H. Hicks Jr. Independent Clements T. Berezoski carried 8.6 percent of the vote. In the Leesburg District, incumbent Democrat Charles A. Bos, who was appointed to a vacant seat last year, beat Republican Georgia White Bange with 54 percent of the vote.

Next year's supervisors will be paid $18,000 annually.

Loudoun voters also approved issuing $15 million in bonds to buy the 357-acre Claude Moore property in eastern Loudoun and turn it into a park.