As endangered species go, neither Herbert E. Harris nor Joseph L. Fisher looks particularly frail.

But Northern Virginia's two Democratic congressmen, who classify themselves as moderates but appear more liberal against Virginia's notably conservative landscape, both have been targeted for extinction.

Their rivals, Stanford Parris and Frank Wolf, Republican challengers for the 8th and 10th District seats respectively, are predicting that President Carter's unpopularity, their own gray-flannel conservatism and heaping portions of money will defeat Harris and Fisher, both of whom are three-term incumbents.

If successful -- and polls in both districts indicate the races are too close to call -- all 10 of Virginia's congressmen will soon share the same conservative perch.

Harris, 54, a skilled and often pugnacious campaigner, and Fisher, 66, a reserved economist who appears uncomfortable touting his own political virtues, will not concede that they, or their breed, are doomed. But officials in both Democratic camps acknowledge that the transience of Northern Virginia's electorate, coupled with the unprecedented $200,000 media campaigns waged by their Republican opponents, have created contests where walkaways once were predicted.

"We've never seen the kind of advertising that Parris has bought this year," says Jack Sweeney, a Harris aide, who now says that he always expected Parris to close a popularity gap that was 15 percentage points last May. "There was never any expectation that Harris was going to clobber Parris or anybody else."

The 8th District race between Harris, Parris and a 25-year-old independent, Deborah Frantz, has been one of the roughest in memory in the Commonwealth, where campaigning is traditionally more genteel.

In the first debate, Harris accused Parris, the man he defeated in 1974, of selling out to Big Oil during his only term in Congress. Parris retaliated by accusing Harris of masquerading as a moderate and supporting economic chaos. Since then the two have slugged it out in vitriolic debates across the district, from Alexandria and southern Fairfax County to Prince William and northern Stafford counties, routinely accusing each other of deception and improprieties.

The race in the 10th District, which includes Arlington, Loudoun and northern Fairfax counties and the cities of Fairfax and Falls Church, has been almost tame in comparison, especially when contrasted to the 1978 campaign between Fisher and Wolf. During that race, which Fisher won by a surprisingly small margin of 9,000 votes, Wolf frequently referred to his white-haired opponent as "grandfather" and irked Fisher enough to provoke the complaint that Wolf was "yipping" at his heels.

Wolf has been more subdued this year, leaving the yipping to a group of teen-aged supporters who call themselves the "Wolf Pack." They cheer wildly at debates, stand at the end of Key Bridge waving Wolf placards during rush hour and clean windshields, leaving behind literature that says, "Now that you can see, vote for Wolf."

Wolf also has been helped by generous contributions from political action committees across the country, including the conservative Christian group Moral Majority. Together they have provided more than half of his $350,000 budget. Parris has been even more successful raising money, particularly as the race becomes closer.

"There isn't a day goes by that I don't open the mailbox and find $5,000 to $7,000 dollars," said a Parris campaign aide, who predicted that the campaign would take in a total of $400,000 after a final media splurge this week.

Both Harris and Fisher have accused their challengers of trying to "buy" the election. The challengers answer that incumbency, and free congressional mailing privileges in particular, more than compensate for their own spending, which is about twice that of their opponents.

Incumbents in both districts have emphasized their constituent service and support of local issues. Harris reminds voters he helped kill a proposed District commuter tax and was instrumental in getting more federal funding for Metro. Both Democrats stress their attempts at reducing flights at National Airport and keeping civil service and Social Security retirement systems separate.

Their challengers have also followed parallel plans of attack -- each trying to hang President Carter around his opponent's neck. Both Parris and Wolf charge that because their rivals supported Carter on approximately 75 percent of House votes this past term, they share responsibility for the shape of the nation's economy.

"I don't call it economy, I call it inflation," says Wolf, a 41-year-old former undersecretary of the Interior Department, congressional aide and, more recently, baby food lobbyist. Though Wolf has never held elective office, he has spent the past five years campaigning at Metro stops, parking lots and trash dumps in repeated attempts to unseat Fisher.

Because an estimated half of all residences in Northern Virginia house either civilian government employes or military ones, the brunt of both campaigns has been fought with those audiences in mind.

In every one of their 16 debates, Parris accused Harris of being soft on national defense, and of voting against another nuclear aircraft carrier and the B1 bomber. And at every debate, Harris answered that he voted for both.

The Parris campaign, however, feels that Harris is vulnerable on his defense posture. Last night a Parris fundraiser was to feature retired Admiral Thomas H. Moorer, the former chairman of the joint chiefs and a critic of Carter's defense policies.

The Harris camp will not concede the military vote. Harris' aide Sweeney argues that Parris directs all of his fire at weapons systems while Harris works to raise military pay and other benefits.

"The 8th District isn't full of missile silos. The 8th District is full of people," says Sweeney. "They're not going to dock a nuclear carrier in Alexandria or build the B1 bomber in Burke."

The primary target of Harris' campaign has been civilian federal employers.

Newspaper ads paid for by the Harris campaign picture the back of a man bearing the headline, "Stan Parris Turns His Back on Federal Workers and Retirees." Harris, who serves on the House Civil Service Committee, claims that Parris would not choose to serve on that committee if elected. Parris counters that he never said he would not take that seat, only that a decision now on which committees to serve on would be premature.

Two unpredictable elements that figure in the outcome of the races are the large number of new voters in the districts and the effect Ronald Reagan will have on the congressional voting.

Current polls show Reagan winning both Northern Virginia districts by a comfortable margin. Parris and Wolf predict that they will benefit from his coattails. But Harris and Fisher both point to the 1976 presidential election as proof that coattails are not that significant in Northern Virginia, which has a history of vote-splitting. Four years ago, Republican Gerald Ford carried both the 8th and 10th Districts, as did Democrats Harris and Fisher.

The other unknown is the political preferences of new voters who have registered in Northern Virginia since the last congressional election. Registrars in both districts call the turnover "unprecedented" and estimate that as many as 35 percent of those voting on Tuesday will be casting ballots in their districts for the first time. In Fairfax County alone, 22,000 voters signed up during the last week of registration.

"It's kind of scary," said a campaign aide to Parris. "I don't know whether they're his people or our people, but they're obviously getting ready to hit somebody on the head with a baseball bat."