Stanford Parris spent $400,000 to win back the Northern Virginia congressional seat he lost six years ago. But probably the most effective piece of political literature in the campaign didn't cost him a cent.

"I think our opponent made a serious mistake," said a top Parris campaign official, referring to a sample ballot handed to voters by Democratic Party workers on Tuesday. The election guide urged them to vote the party line -- Carter and three-term Rep. Herbert Harris. "He [Harris] did more with that piece of paper than we did with all of our advertising," said the aide.

For six months, Republican challengers in both Northern Virginia's 8th and 10th congressional districts tried to place Harris and incumbent Rep. Jospeh Fisher on the bridge of President Carter's sinking political ship. Both men, seeking fourth consecutive terms, refused to go aboard. On Tuesday they were dragged under.

"I think you've got to say that Ronald Reagan's coattails had a very significant effect on this election," said Jack Sweeney, an aide to Harris. The 8th District, which includes Alexandria and Prince William County, southern Fairfax and northern Stafford counties, went for Reagan by 42,000 votes.

Returns in the 10th District were equally lopsided for Reagan, which was blamed by Fisher workers yesterday for the 4 percentage point loss by Fisher to Frank Wolf.

"There were an awful lot of people wh o didn't stop to look at what Fisher and Harris had done. They saw the commercials calling for change and they listened to Reagan," said Dottie Schick, Fairfax County's Democratic chairman.

But while Reagan's sweep of Northern Virgina was being credited, or blamed, for the Democratic debacle and the defeat of Virginia's last two liberal congressmen, other factors contributed to the upsets.

Deborah Frantz, the 8th District's 26-year-old independent candidate, apparently played the spoiler role in that three-way contest. Harris lost to Parris, the man he beat in 1974, by less than 1 percentage point. Frantz, who campaigned to repeal marijuana laws, captured 3 percent of the votes cast.

Conceding she supported Harris on most other issues, Frantz would not apologize for the victory she apparently denied him. Campaign officials for Harris also refused to blame Frantz for the loss, perhaps out of concern about being identified with a dope smoking constituency.

Pat Sullivan, however, eagerly credited Frantz for the Harris defeat.

A onetime Harris campaign manger, Sullivan, who professes intense dislike for his former boss, claimed to have mastermined Frantz' spoiler role in the race.

"I just wanted to get rid of him, I was tired of getting those newsletters in my mailbox," said Sullivan. "I figured if we could cut into him by a small margin, he'd be a loser."

Both Frantz and her campaign manager disputed Sullivan's claims. Though he helped place her name on the ballot and printed campaign materials at his own expense, Sullivan, they said, did more talking than politicking.

In the 10th District, encompassing Arlington, Loudoun and the northern half of Fairfax County, Democrats attributed Fisher's loss more to voter dissatisfaction with Carter than Wolf's campaign promises.

Wolf "promised no inflation and sunshine 24 hours a day," said Democrat Ira Lechner, a former state delegate from Arlington. "Two years or four years from now we'll shove those promises right back down his throat."

Though Harris and Fisher won ringing victories in their home bases of Alexandria and Arlington respectively, both were beaten by lopsided returns in Fairfax County and in the rural areas of Prince William and Loudoun counties.

Prince William, which Harris had carried in his three previous elections, sided with Parris by almost 2,000 votes. Parris officials credited that turnaround to a mailing of 41,000 letters signed by Reagan asking voters to "make America great again" by supporting Parris.

Tenth District Democrats disputed the prevailing view of some political observers that Tuesday's results signal an end to nearly 30 years of progressive activism. But voting figures give signs of potentially serious problems for future Democratic office seekers.

Traditional ticket splitting, still evident in Arlington and Fairfax, did not hold in Loudoun County. Democrats acknowledged that new, young voters driven to the developing outlying area in search of moderate housing voted straight Republican tickets. Republicans also said traditional Democrats among the district's elderly and "empty nesters" sought a conservative haven from inflation and high taxes.

Despite Tuesday's results, Democratic officials in both districts took heart that Harris and Fisher lost by relatively small margins when compared to Reagan's crushing triumphs in the same areas.

"It was a hell of a strong Republican turnout . . . a lot of people were clobbered," said Harris aide Sweeney. "When you have a 42,000-vote difference in the presidential election and we came within 1,100 votes, we feel very proud."

Lechner agreed. "If you're looking toward the future, these districts are marginal for Democrats and Republicans. In each case it was small but perceptible shifts from one column to the other" that made the difference.

But not all Democrats found reason for optimism. Following Harris' stunning defeat Tuesday, one teary eyed campaign worker said, "Let's build a coalition." Responded another, "Hell, let's build an ark."