Republicans and Democrats agree that the two GOP congressional nominees from Northern Virginia could pose serious threats to their opponents, both Democratic incumbents, if the GOP can make good on its promises of party unity.

But the Republican threat showed signs of tarnish on election night, as mainline Republicans and the so-called "new right" continued the bitter squabble that divided voters in the GOP primary last week.

Robert L. Thoburn, the ultraconservative minister who lost his primary bid against former congressman Stanford E. Parris, had hardly conceded the race before he was deriding those in Alexandria who had opposed his candidacy. In a television interview the night of the primary, Thoburn labeled those opponents "monkeys." At the same time, Parris supporters privately were heaving sighs of relief over the demise of "the Koolaids" -- their word for Thoburn backers, drawn from the example of the People's Temple massacre in Guyana.

Northern Virginia Republicans are gleeful about the economic and foreign problems of President Carter, and they are confident a Reagan landslide in November will propel their nominees -- Parris in the 8th District and Frank Wolf in the 10th -- into Congress.

Democrats, for their part, have moved quickly into a defensive posture around incumbents Joseph L. Fisher (10th District) and Herbert E. Harris II (8th District), stressing their records and emphasizing that this fall's congressional elections will not hinge on Carter.

"I'll run my own campaign just as I always have," says Fisher, whom Wolf has called "part and parcel" of the Carter administration.

Fisher narrowly defeated Wolf two years ago, and expects his voting record to be a key GOP issue in November.

"Sure, I support Carter; I'm a Democrat," Fisher said. "I vote with him frequently, but quite frequently I don't."

He pointed out, for example, that he had broken with Carter over the president's proposal of a 10-cents-a-gallon gasoline tax.

Harris, who almost never mentions his name and Carter's in the same breath, dismissed the presidential "coat-tail" theory.

"That kind of thinking is totally inappropriate in the 8th District," Harris argued. "People there want to know what you're for and what you've done for them, and no candidate can get away with hiding behind another one."

Harris gave Parris little time to savor his primary victory before blasting his old foe, alleging misuse of congressional office funds and other improprieties.

Parris served one term in Congress before being ousted by Harris in 1974.

"I've been wanting to know the answers to some questions for a long time," said Harris. Specifically, he said, he wants to know what happened to constituent service files and funds from an office stationery account, which Harris says should have been returned to the Treasury.

Parris accused Harris of hurling the first of "a lot of dreamed-up, trumped-up smokescreens." He said he turned over all his files when he left office and had used no office funds improperly.

The charges, Parris said, are attempts to detract from Harris' voting record on inflation, military preparedness and energy matters.

"He's riding with Jimmy Carter all over him like a wet Indian blanket," said Parris, "and that is a very havey burden."

Emilie Miller, a member of the state Democratic Central Committee and former chairman of the Fairfax County committee, predicted Carter would not have much impact on the Northern Virginia congressional races.

"Herb and Joe won in 1976 even though Carter didn't win in Northern Virginia," she pointed out. But she added that Harris and Fisher were prepared for a tough fight.

"They never have easy races," Miller cautioned. "They always have to run as if it's the first time because of all the transients moving in and out."

Ray Colley, Democratic Party chief in the 8th District, said Wolf and Parris would be challenged to come forth with solutions to any problems they point out.

After the primary, many Republican officials claimed Parris' victory as a gain for party unity, and predicted many Thoburn supporters would come back to the party mainstream.

"We're walking on clouds," said Fairfax Supervisor Marie B. Travesky (R-Springfield). "We're so delighted I can't tell you how happy we are."

Travesky said the 60-40 vote for Parris showed many area Republicans were "fed up" with what she called the "attempted takeover of the Republican Party by the extremists."

But Thoburn supporters charged privately that party moderates were attempting to drive more conservative members from the GOP. A close political ally of Thoburn, who asked not to be named, charged that Thoburn had been unfairly branded "ultraconservative" by moderate Republicans who were struggling to keep up with a perceived shift to the right among the voters.

"They're in the middle and they want to be seen as more conservative," the Thoburn associate said. "So now they're calling themselves moderate conservatives and branding the real conservatives as ultraconservative, new-right people."

Party regulars rejected claims that Thoburn had been unfairly categorized.

"It's not a question of somebody labeling him ultraconservative," said state Del. Warren E. Barry (R-Springfield), chairman of Parris campaign committee. "Bob has maneuvered himself into that position and taken great pride and pleasure in it."

While one Thoburn supporter said he expects this latest defeat to end Thoburn's career as a candidate, many Republicans and Democrats were certain he would continue in elective politics.

"Apparently he feels he has a message from God to keep trying," said Democrat Miller.

Thoburn, still smarting after a campaign in which he said the media and mainstream Republicans were aligned against him, refused to discuss the matter.

"You stink and your newspaper stinks," he said.