Democrat Katherine K. Hanley said yesterday she will not seek reelection as Board of Supervisors chairman in Fairfax County in November, clearing the way for a run for Congress in Virginia's 8th District next year.

Hanley's decision not to campaign for a third four-year term on the county board could reorder Fairfax politics by giving the GOP, which did not field a candidate against her four years ago, its best shot in a decade at the top job in the region's largest local government.

Hanley, 60, said she could not run for reelection while she explores a race against Rep. James P. Moran Jr. in a Democratic primary in 2004. "It's time to move on," said the former schoolteacher, a fixture in Northern Virginia politics for close to 20 years. "There are other opportunities that are very timely for me to consider. It all fits together at this time."

She said she would do her fellow supervisors and constituents a disservice if she were to win reelection this fall only to turn around in January and campaign for Congress -- a race that would require a prodigious fundraising effort.

Her announcement came less than two weeks after Moran ignited a bipartisan furor with remarks that Jews were pushing the nation into war with Iraq. She could give the Alexandria congressman, who has been elected seven times, his greatest and most expensive challenge. Democrat Leslie L. Byrne, a state senator and former congresswoman from Fairfax, also inched closer yesterday to running against Moran, saying she is filing papers for an exploratory committee to assess a primary challenge.

Washington lawyer Jeremy Bash, a foreign and defense policy aide to Al Gore's 2000 presidential campaign, announced yesterday that he, too, is seriously considering challenging Moran for the Democratic nomination.

Moran, who has said repeatedly in the last week that he intends to run for an eighth term, declined through a spokesman to comment on Hanley's announcement. "I will continue to focus on serving in the United States Congress and my constituents in the 8th District," he said in a statement.

Hanley lives in Reston, which became part of the 8th District two years ago. The district still includes Moran's strongholds of Alexandria and Arlington, but redistricting has extended it west. About half his constituents now live in Fairfax County.

Fairfax Democrats moved quickly yesterday to try to maintain their party's dominance on the Board of Supervisors, choosing Gerald E. Connolly, the Providence District supervisor, to run as Hanley's replacement for board chairman Nov. 4.

"To be chairman of this board, I think it's important to have served on it," Hanley said. "Gerry is by far more qualified than any of the opponents I've heard about."

The departure of an incumbent who had scared away serious challengers left Fairfax Republicans gleeful at the prospect of a race for an open chairman's seat and for Connolly's seat. Emboldened by mounting anger from homeowners hit with soaring property tax bills and by last fall's defeat of a sales tax increase for transportation projects, the GOP is preparing for its first contested primary race for board chairman in 12 years.

"I've got an open seat and two heavy hitters standing in the batter's box," said Eddie Page, the Republican county chairman. Two candidates, School Board Member Mychele B. Brickner and John F. "Jack" Herrity, a former Board of Supervisors chairman, will battle for the party's nomination in a June primary.

"I daresay I don't think I could walk anywhere in Fairfax County and find anyone who is happy paying more property taxes," Page said. Several other supervisors, including one incumbent Republican, Elaine N. McConnell (Springfield), face GOP challengers with anti-tax messages. Supervisor Michael R. Frey (R-Sully), who opened the door last week to a race for chairman if Hanley pursued a campaign for Congress, said yesterday he had not "done the homework that needs to be done" and would not run.

Brickner and Herrity were subdued in their reaction to Hanley's announcement.

Herrity, 71, said his candidacy for board chairman would focus on the same issues -- high property taxes and the Democrats' "failure to build adequate roads" -- regardless of who his opponent is.

Brickner, 52, who plans to formally announce her candidacy Tuesday, said an open seat "makes it a bigger opportunity" but declined to say more. As an at-large School Board member, she has been a leading voice of conservative parents, pushing for restrictions on sex education, opposing books with graphic sexual content and violence and fighting efforts to broaden protections for gays and lesbians. She has said little about her campaign platform for board chairman.

Other Republican leaders were more blunt about their party's changed prospects.

"Any challenger on the Democratic side will run up against the tax issue," said Supervisor Stuart Mendelsohn (R-Dranesville), who is not seeking reelection. "Kate's departure changes the race. It makes this a fascinating year."

Both parties are scrambling to recruit a candidate for Connolly's seat.

Connolly, 52, is among the county's most vocal and ambitious supervisors, stopping at civic meetings, government hearings and school events most days and nights and serving on numerous regional boards. As state cuts and a slow economy have forced Fairfax to rely increasingly on real estate tax revenue to buoy services, Connolly has led Fairfax Democrats in criticizing legislators in Richmond for not doing more to support local spending. While incumbency on the board will give him advantages in the chairman's race, Connolly has not run countywide and is untested in Fairfax's heavily Republican western end.

Connolly plans to make a formal announcement this month, and he was in full campaign mode yesterday, calling Brickner a "rigid ideologue."

"Fairfax voters consistently have supported mainstream leaders," he said.

Hanley will leave office at the helm of a county that grew to more than a million residents on her watch. In her nine years as chairman -- she won in a special election in 1995 -- Fairfax continued its transformation from a bedroom community to a major job center, riding a technology boom and bust. As a county representative on the Metro board, Hanley has championed efforts to bring rail to Tysons Corner and Dulles International Airport. She led the county board in expanding human services to vulnerable residents, largely by giving nonprofit groups a more active role.

"Kate has been such an integral part of the county's leadership for so long, it's hard to picture Fairfax without her as a key player," said Supervisor T. Dana Kauffman (D-Lee).

Staff writer Spencer S. Hsu contributed to this report.

Katherine K. Hanley (D) has been Board of Supervisors chairman since 1995.