Support for both major candidates has eroded in Northern Virginia's hotly contested 8th Congressional District race, apparently because voters are increasingly disgusted with the nastiness of the campaign, according to a Republican pollster.

Republican Rep. Stanford E. Parris released a poll yesterday that shows him leading, 44 percent to 36.5 percent, over his Democratic challenger, former representative Herbert E. Harris II. Parris released the poll, which has a six-point margin of error, to contradict an Associated Press/WRC-TV survey that last week gave Harris an eight-point edge.

The Parris poll showed, however, that support for the Republican incumbent has dropped from 50 percent to 44 percent since February, while Harris has dropped from 39 percent to 36.5 percent. Parris' spokesman said his pollster, V. Lance Tarrance of Houston, attributed the decline -- and the increasing number of undecided voters -- to both candidates' negative campaigning.

"The only thing I get is complaints: 'Why don't they stop putting each other down?'" state Del. Frank Medico, a Republican who has been campaigning door to door in the Mount Vernon section of Fairfax, said recently. "They say it's the same old Herb, the same old Parris, and they don't like it."

Harris said yesterday the latest Parris poll, which interviewed 200 likely voters between Oct. 11 and Oct. 14, is "pathetically inaccurate and demonstrably so."

In a related development yesterday, Harris was endorsed by four major environmental groups, including the Sierra Club and the Friends of the Earth. Harris accused his opponent of "surpassing James Watt in his zeal to reduce funding for environmental protection programs."

Parris and Harris know and dislike each other thoroughly as they face each other for the third time in a district that includes Alexandria and parts of Fairfax, Prince William and Stafford counties. Harris unseated the Republican in 1974 and then served three terms before being unseated by Parris in the 1980 Reagan landslide.

Parris set the tone for the campaign last month when he began airing radio commercials about Harris' "hot air balloon" that accused Harris of voting to fund a bureaucrats' junket to the Virgin Islands and a study of the mating habits of ducks. Parris has called the rematch "a referendum on Herb Harris."

He also continually accuses Harris of disguising his liberal feelings under a layer of rhetoric about balanced budgets and fiscal responsibility. "Sending Herb Harris to Congress to protect small business is like hiring Evel Knievel to run the Fairfax County driver ed program," Parris said last week in a variation of a line he uses at every debate.

Harris, in turn, is running television commercials that accuse Parris of betraying federal workers, military retirees and Social Security recipients. The supposedly dismal voting record is imposed on a picture of Parris that makes him look "like he just escaped from a gorilla cage," according to Board Chairman John F. Herrity, a Parris supporter. (A Harris spokesman said the picture comes from The Almanac of American Politics 1982.)

Herrity, who ran unsuccessfully against Harris in 1978, also said the voter disenchantment with both candidates could depress turnout on Nov. 2 or help Citizens Party candidate Austin Morrill. The 33-year-old Morrill, who was virtually unknown in earlier surveys, was not mentioned in the latest Parris poll, which shows nearly 20 percent of the voters still undecided.