Tell a casual observer of Washington-area politics that there is a knockdown, drag-out brawl for Congress in Northern Virginia and he will deduce that it is in the 8th District.

That is where Democrat Rep. Herbert E. Harris II is being challenged by his old nemesis from the Fairfax County Board of Supervisors, Republican John F. (Jack) Herrity.

But if the old pol stops there, he will be only half right.

Another bare-knuckles fight is taking place upstream in the 10th District, where mild-mannered Democrat Rep. Joseph L. Fisher is trading insults with a brash young Republican challenger named Frank Wolf.

Harris and Herrity may have been more restrained than usual by the presence of the third candidate in the race, independent nominee Charles Coe, who came between them literally and figuratively by sitting between them on the set.

Nontheless, Herrity managed to accuse Harris of "errors" and "misleading" statements in his campaign commercial, with Harris replying that Herrity was outspending him 3-to-1 for television time.

The intensity of the races was illustrated during interviews yesterday for a public television program broadcast last night on Channel 26.

Fisher and Wolf continued harranguing each other after the cameras were turned off in the Arlington studios of WETA-TV yesterday afternoon.

While they were on the air, Wolf pointed a finger at Fisher and said "your record is so bad that these people [big business that are contributing to Wolf's campaign] would like to see you taken from Congress."

Fisher's version of why numerous political action groups are supporting Wolf is that "I'm on the Ways and Means Committee, which writes the taxes and oh, how those big businesses would like to get rid of me."

Wolf shot back that Fisher's complaint was "very inconsistent . . . with the fact that your own finance director wrote to those same [organization] seeking contributions. Isn't that right?"

When Fisher said, "Right," Wolf said, "How can you criticize me for taking that money?"

When the debate turned to the Equal Rights Amendment, Fisher accused Wolf of "squirming and wiggling around" trying to explain away his opposition, while Fisher said simply "I'm for it."

Wolf, a 39-year-old lawyer and former congressional aide, dragged out several newspaper clippings in attempts to support various accusations against Fisher.

He waved a year-old story that Wolf said showed that Fisher discriminates against women on salaries paid in his congressional office. Fisher answered that 13 of 16 workers on his Capitol Hill payroll are women. Fisher said the three men have higher average salaries because "two of them are highly trained lawyers."

Wolf said he has women campaign and finance directors and if elected "will probably have a woman administrative assistant."

The 10th District segment of the hour-long program opened with Wolf saying that if voters in Arlington, northern Fairfax and Loundoun counties "truly knew how he (Fisher) voted, there is no way he would ever be elected."

Aware that for two decades voters in the district elected Joel Broyhill, a conservative Republican, Wolf charged that Fisher is "avoiding what he truly is, a solid liberal . . . with big labor support.

"You have a right to be liberal, but you have a duty to tell you constitutents how you vote," Wolf added.

Fisher, a 64-year-old economist with solid liberal credentials, replied that his votes "are a matter of public record," and urged Wolf to "get to a more constructive note."

The two-term representative said less than 10 percent of his $110,000 in campaign money came from big labor while 85 percent came from more than 2,550 individuals who live within the district. Fisher compared that with figures he said showed that more than half of Wolf's $148,000 in contributions came from organizations based outside the district.