Northern Virginia's two liberal Democratic members of Congress last night defeated their well financed and conservative Republican challengers.

In the bitterly contested 8th District, Rep. Herbert E. Harris II soundly beat John F. Herrity in Alexandria and in Prince William County. Herrity, chairman of the Fairfax County Board, managed only a narrow victory in Fairfax County where he needed a large majority to win the seat.

In the neighboring 10th District, Rep. Joseph L. Fisher surged to a strong and early lead across Arlington, Fairfax and Loundoun counties. Wolf, who had campaigned on the claim that Fisher was too liberal for Northern Virginia, failed to win Fairfax County, an area he had counted on for conservative support.

Across the state, the 6-to-4 Republican edge over Democrats in the congressional delegation appeared safe, except in the "Fighting 9th" District, where Republican incumbent Rep. William C. Wampler faced a strong challenge from a farmer Charles Clark.

In the 1st District, which sprawls south of Fredericksburg on both sides of Chesapeake Bay, freshman Republican Rep. Paul Trible appeared to have defeated Democrat Lewis Puller, a Vietnam veteran who campaigned from a wheelchair.

In the 3rd District, which surrounds Richmond, incumbent Democrat David E. Satterfield soundly defeated U.S. Labor Party candidate Alan R. Ogden.

IN the 7th District, which includes parts of Fauquier and Stafford counties, incumbent Republican Rep. J. Kenneth Robinson defeated the Democratic challenger, state Del. Lewis P. Fickett, a political science professor.

Statewide, the two House races in Northern Virginia attracted the most attention, with two well-financed Republicans taking on two liberal Democratic incumbents.

Republican challenger Herrity often accused Harris of being "the biggest spending congressman in Virginia" while Wolf tried to paint Fisher as too liberal for the voters of the 10th District.

Herrity attempting to capitalize on voter unrest over inflation and government spending, repeatedly promised to have "the guts to say no" to new spending schemes if elected. Wolf pounded away at Fisher by claiming that Fisher failed to vote in Congress the way he talked to his constituents.

In the last week of the campaign, Herrity aroused Harris' anger by mailing a campaign letter that accused Harris of voting in Congress against penalties for child pornographers and in favor of tax spending on homosexuals.

Harris called the charges "lies" and said they constituted a "last-minute smear campaign.

The two-term incumbent elected in 1974 along with 75 other so-called "Watergate babies," argued in more than 25 debates with Herrity that the issues in the campaign were support for the Metro system and the fight against any commuter tax for Northern Virginians.

Harris led a committee vote in Congress last summer that stopped a commuter tax bill. He has been a highly visible Metro supporter for 10 years. Harris accused Herrity, who also said he supported Metro despite his long record of opposition to regional mass transit, of changing his tune because of growing public support for Metro.

In the neighboring 10th District, Wolf, who outspent Fisher nearly 2 to 1, made a habit in debates and television advertising of accusing his opponent of duplicity and misrepresentation. Fisher, 64, who coul pass as a kindly uncle to the 39-year-old Wolf, often appeared uncomfortable and upset by his challenger during their appearances together.

Fisher, who also was elected to Congress with the "Class of 74," said that "coping with Wolf's yipping and yapping at my heels" detracted from serious issues in the campaign such as "sensible" tax reform and energy conservation legislation.

The incumbent accused Wolf of relying on outside contributions to support his campaign. Much of the $190,400 that Wolf received came from conservative political groups and Republican committees.

Elsewhere in Virginia, Republicans G. William Whitehurst in the 2nd District, Robert W. Daniel Jr. in the 4th District, and M. Caldwell Butler in the 5th District, and Democrat W. C. Daniel in the 4th District were elected without opposition.