Two years ago, Democratic businesswoman and community activist Leslie L. Byrne won the 38th District seat in Virginia's House of Delegates after a campaign that attacked the Republican incumbent as ineffective.

Now, Byrne, 40, faces the same criticism from her Republican challenger in the Nov. 3 election, lawyer and civic leader A. Strode Brent Jr., 45.

"I don't see anything she's really done that's remarkable," Brent said in an interview at his Fairfax City law office, across the street from the county courthouse where he worked as an assistant commonwealth's attorney before going into private practice a decade ago. Brent, making his first try at political office, calls himself the "mainstream candidate," and describes Byrne as a liberal.

But Byrne, whose campaign headquarters are in her high-tech consulting firm in Falls Church, describes Brent as a political tyro waging a purely negative campaign. "President of the swim club is not a prerequisite for elected office," she said. " . . . He's a guy who's come from nowhere, doing nothing." Byrne calls herself a "solid Virginia Democrat" with a "remarkably successful" legislative record.

The district they seek to represent is a 16-square-mile area in east-central Fairfax County, with about 31,000 registered voters. It is bounded by Little River Turnpike to the south, Prosperity Avenue to the west, Arlington Boulevard to the north and Leesburg and Columbia pikes to the east.

A built-up melange of middle-class houses, shopping centers and office buildings, the district has swung between Republicans and Democrats.

To win, observers say, Brent must overcome not only Byrne's advantage of incumbency and her larger pocketbook -- $47,500 to his $25,000 to $40,000 -- but also potentially unsettling ripples from the troubled reelection campaign of Republican Board Chairman John F. Herrity.

"He's a very good candidate," County GOP Chairman Jim Swinson said of Brent. "Whether or not he can overcome the incumbency depends on whether he can get his name out."

County Democratic Chairman Harris N. Miller said he believes that Byrne will be reelected: "Leslie has done an outstanding job . . . . As a freshman legislator, maybe she needs a little more push {to win}, but her record certainly stands on its own."

Byrne is certainly pushing; as of last week, she said she had knocked on 4,000 doors. Some wonder whether her opponent's low-key style -- Brent conceded a few months ago, for example, that Byrne has been "fairly successful" as a delegate -- will triumph over Byrne's more aggressive personality.

Brent already has scored one political upset, edging former incumbent Gwendalyn F. Cody (defeated by Byrne in 1985) by fewer than a dozen votes out of nearly 2,700 cast in a June primary that was billed as a victory for the party's moderates over its conservatives. He has allied himself with popular County Supervisor Thomas M. Davis III (R-Mason).

Byrne talks of her solid alliance with the Democratic Party, including a recent endorsement by State Attorney General Mary Sue Terry, and boasts that eight of the 13 pieces of legislation she has introduced in her two years in office have been enacted into law. They include statutes raising parental liability for their children's vandalism to $750 from $500; requiring the state Treasury Board to approve all proposed state bond issues, and increasing property tax relief for the elderly and the handicapped.

Brent dismissed most of Byrne's legislative efforts as minor, and disputed whether one of them, her much-touted "truck cover" bill, would work. The bill provides certain counties, including Fairfax, with the authority to regulate the tracking of mud and debris on public highways.

Byrne said the bill allows local officials to enact ordinances requiring truckers to cover their loads; Brent cited opinions by Terry and Fairfax County Attorney David Stitt that the bill does not allow that. Byrne responded that the only way to find out would be to pass such an ordinance and allow the matter to be settled in court.

The key issue in the district, both candidates agree, is growth and transportation. Byrne said the state must give localities the power to manage growth better, including allowing them to enact public facilities ordinances tying zoning approval to construction of needed public services. Brent said he is not certain whether such an ordinance is a good idea. County supervisors have endorsed such an ordinance, but the state legislature has rejected proposals to give local officials the power to enact one.

Byrne was elected after spending 1984 and 1985 as a legislative aide to Del. Vivian Watts, now state transportation secretary. She also headed the now-defunct Fair Campaign Practices Commission and has been active in the League of Women Voters, Metropolitan Housing Authority and other civic activities including the PTA. She and her family have lived in Falls Church since 1973.

Brent, a graduate of Annandale High School and the University of Virginia, has been president of the Lake Barcroft Civic Association, a member of the county Tenant Landlord Commission and active in the county bar association.