Del. Richard H. Black of Loudoun County, one of the General Assembly's most conservative members, was defeated yesterday, helping Democrats win three key House of Delegates seats in Northern Virginia.

The victories will not significantly change the balance of power in Richmond, however. When the General Assembly convenes in January, there still will be a Democratic governor and a House and Senate dominated by the Republican Party.

Democrats yesterday added one seat to the 38 they hold in the House. Republicans, who now hold 59 seats, helped offset their losses in Northern Virginia by winning two seats, on the Northern Neck and in southwest Virginia.

Black, a four-term incumbent, lost to upstart Democrat David E. Poisson of Sterling, who challenged the Republican's anti-tax stand and suggested that his focus on such socially conservative issues as abortion came at the expense of the needs of the fast-growing district in Washington's outer suburbs.

Black blamed his loss on voter dissatisfaction with Republicans nationally. "My guess is this is the way the political winds were going. We were campaigning into a headwind," he said last night.

In Prince William County, Del. Jeffrey M. Frederick (R) defeated Hilda M. Barg (D), a veteran county board member, in a district that has long been held by Republican state legislators but which voted for Sen. John F. Kerry (D) in last year's presidential election.

In another closely watched race for an open seat, in the 37th District, Democrat David L. Bulova, son of Fairfax County Supervisor Sharon S. Bulova, defeated former Fairfax City mayor John Mason (R). Both parties made a priority of winning the seat, which is being vacated by J. Chapman Petersen (D).

At a hotel in Tysons Corner, Bulova appeared before his supporters and said the election was about the things voters cared about: education and transportation. "We knocked on 11,000 doors and made that personal connection. I intend to take that feedback to Richmond."

In another race for an open seat, Democrat C. Chuck Caputo of Oak Hill defeated GOP youth minister Chris S. Craddock of Fairfax in the 67th District of western Fairfax and eastern Loudoun counties.

Craddock was one of four defeated candidates who had been critical of gays and lesbians during the campaign. The others were Black, Ron M. Grignol Jr. of Fairfax and Del. Bradley P. Marrs (R-Richmond).

Marrs, who was losing late last night by fewer than 50 votes, said he will demand a recount, House sources said.

Del. Brian J. Moran (Alexandria), chairman of the House Democratic Caucus, who was reelected, said: "Our message has resonated in Northern Virginia. We've been saying it all along. I could feel it."

In other races, Democrat Dave W. Marsden of Burke defeated Republican anti-tax candidate Michael J. Golden of Fairfax in the 41st District race to replace James H. Dillard II (R-Fairfax), a Republican.

One bright spot for the GOP in Northern Virginia was Del. David B. Albo's victory over Democrat Gregory A. Werkheiser in a hard-fought campaign in the Springfield area.

Political observers said House candidates were aided by the strong showing of Democratic Lt. Gov. Timothy M. Kaine in Northern Virginia, especially in Fairfax, in his defeat of Republican Jerry W. Kilgore for governor.

"Without a doubt, we saw Tim Kaine's coattails help the down-ticket House candidates," said Mark J. Rozell, a professor of government at George Mason University. "What we saw was an anti-Republican vote in Fairfax County, and I think President Bush coming into the state may have created a counter-mobilization vote in Northern Virginia."

Outside the Washington suburbs, Del. Thomas D. Gear (R) defeated independent Randy A. Gilliland in a closely watched race in the Hampton area. In southwest Virginia, Del. W.B. "Benny" Keister (D-Pulaski) lost to Republican Anne B. Crockett-Stark.

Democrats, who picked up three seats in the 2003 election, set an ambitious goal of snaring three more. Republicans focused their efforts on winning seats downstate and on resisting the Democratic charge in Northern Virginia.

All 100 House seats were on the ballot yesterday, but only about half were contested, including 18 in Northern Virginia, where six Republican incumbents were unopposed (as were four Democrats). Republicans also hold a 24 to 16 edge in the state Senate, although none of those seats was up for election.

House Democrats campaigned on a message of "keeping a good thing going," referring to Gov. Mark R. Warner's popularity and support by most Virginians for the $1.5 billion tax package he helped push through last year. On campaign literature and in cable TV advertising, candidates talked up their association with the governor.

In addition, the well-funded Democratic candidates tried to exploit Bush's low approval ratings and a difficult couple of months for congressional Republicans. House Democratic leaders and candidates tried to appeal to voters by contrasting Virginians' general happiness with state government against apparent voter dissatisfaction with national politics.

Republicans responded with promises to curb state services for illegal immigrants, fund transportation initiatives without raising taxes and continue to phase out the state's hated car tax. The pledges were part of a GOP attempt to link candidates' themes statewide.

Republican leaders said they were worried about the impact of national politics on delegate races, particularly in Northern Virginia.

"The trends seem to be heading our way in the majority of the state, but we'll have to see how we can reverse our fortunes in Northern Virginia," House Majority Leader H. Morgan Griffith (R-Salem) said.

Democrats ran against lawmakers who opposed the $1.5 billion tax package that raised money for education, health care and public safety. From the beginning, they eyed the prospect of gaining seats on key House committees.

Republicans were guardedly optimistic throughout the campaign, aware that they might lose seats, particularly in Northern Virginia. Several operatives said privately that they would be pleased if the party maintained the status quo.

The stakes were high for the GOP: After losing the three seats in 2003, the party lost a Norfolk special election in 2004.

Democrat David L. Bulova celebrates with daughter Josette, 6, after his victory speech. He defeated former Fairfax City mayor John Mason (R) in the 37th District. Democrat C. Chuck Caputo is all smiles as he prepares to give his victory speech in Tysons Corner. Caputo defeated Republican Chris S. Craddock in the 67th District.