James K. "Jay" O'Brien Jr., a conservative Republican who has represented western Fairfax County in the House of Delegates for a decade, scored a comfortable victory yesterday against a Democratic political newcomer in a special election to fill the vacant 39th District seat in the Virginia Senate.

O'Brien jumped to an early lead over Rosemary M. Lynch, a school activist from Franconia, and steadily pulled away in the contest for a seat left open by the August retirement of the longtime Democratic incumbent.

The victory strengthened the GOP's hold on the Senate to 23 of the 40 seats.

"I was running on my experience, and it worked out," O'Brien said last night. He said his top priority in the Senate will be to seek a new funding formula to channel tax money to Northern Virginia to pay for transportation projects after the defeat of the effort to raise the sales tax for that purpose.

The special election was largely overshadowed by the Northern Virginia sales tax referendum, an issue that sharply divided the two candidates.

O'Brien, 50, who lives in Clifton, sided with opponents of the tax increase of a half-cent per dollar, though he had voted in the General Assembly in favor of holding the referendum. He argued that instead of raising the tax regionally to fund transportation projects, Northern Virginians should fight to change "an unfair funding formula that takes our taxes and sends them downstate." He said he would work to form a voting bloc of legislators to ensure a more equitable distribution of tax revenue.

Lynch, 55, helped found Citizens for Better Transportation, a group of civic, political and business leaders that stumped hard for the sales tax measure. She argued that the new money would help relieve Northern Virginia's notorious traffic congestion.

The battleground in the race -- the only General Assembly contest on the ballot in Northern Virginia -- was a newly drawn Senate district straddling Fairfax and Prince William counties. To accommodate Northern Virginia population growth identified by the 2000 Census, the Republican-controlled assembly relocated southwestern Virginia's 39th District, held for 29 years by Sen. Madison E. Marye (D) of rural Montgomery County. Marye decided to retire in August but challenged the special election in court on grounds that it disenfranchised his area's voters. A judge dismissed the lawsuit last month.

The redrawn 39th stretches across Fairfax from Clifton to Franconia and south into Prince William's Occoquan watershed. The district has a slight Republican majority but is considered a swing district.

Although he acknowledged the dominance of the sales tax issue in the election, O'Brien said he did not believe his political fate was tied to it.

At Centreville High School yesterday, Laurie Horanburg, 50, a registered nurse, said she voted for O'Brien because "I'm very much aligned with his conservative values."

And John Marion, 41, an evangelical missionary, said he shared O'Brien's opposition to the sales tax increase. "I don't have any confidence in the way the government uses the tax dollars we give them now," Marion said. "It makes no sense to give them more of my money."

Lynch, a breast cancer survivor and mother of two, was seeking her first public office. She attacked O'Brien as too conservative for the district and denounced his opposition to gun-control measures.

O'Brien, who has represented the 40th District in the House of Delegates for the past decade, is a West Point graduate and was a Reagan administration appointee in the Department of Health and Human Services. He is a father of five, runs an office products business with his wife and serves as a colonel in the Army Reserve.

James K. "Jay" O'Brien Jr. said his top priority will be to seek a new funding formula to channel money to Northern Virginia for transportation projects.