A 33-year-old nuclear freeze activist has collected enough signatures to qualify as a third-party candidate for Congress in Northern Virginia's 8th District, bringing cheer to the hard-pressed reelection campaign of incumbent Rep. Stanford E. Parris (R).

Austin W. Morrill Jr., a Vietnam veteran and recent George Mason University graduate, will run on a Citizens Party ticket, with a moratorium on the production of nuclear weapons his chief issue. Republican strategists said Morrill is likely to take some votes away from former Democratic representative Herbert E. Harris II in his rematch against Parris, his longtime Republican antagonist.

"I welcome him into the race, obviously," Parris said yesterday. "Any impact this young man has on the campaign has to be contrary to Herb's campaign."

Parris has been elected twice to Congress, both times winning less than 50 percent of the vote as independent candidates have attracted votes that Democrats had expected to win.

In 1980, Parris turned Harris out of office by little more than 1,000 votes, as independent Deborah Frantz, who called for the legalization of marijuana, collected about 5,000 votes.

Harris, who was a congressman from 1974 until 1980, downplayed the significance of Morrill's candidacy.

The Democrat said Parris would like to deflect attention from the economy with the nuclear freeze issue. "Stan would prefer to talk about nuclear strategy, space programs, or anything other than the economic situation as it impinges on the people of Northern Virginia," he said.

Parris opposes the freeze, while Harris declined to say how he would vote on a freeze resolution. The resolution "provides simplistic answers to what is a very complex, albeit important, issue," Harris said.

Harris won a majority of the vote in 1976, despite the presence of an independent candidate campaigning against "corruption and decadence" in the federal government, and in 1978, despite independent opposition from a sales representative for a pest control company. Only Harris and Parris ran in the bitter, name-calling 1974 race that began their feud.

Morrill has not yet been certified a candidate by the State Elections Board, but elections officials in Fairfax County and Alexandria said yesterday they had validated 1,282 registered voters' signatures on Morrill's petitions, more than the 1,082 needed to gain a spot on the ballot. The 8th District covers Alexandria, southern Fairfax and parts of Prince William and Stafford counties.

Morrill, whose party fielded environmentalist Barry Commoner as a presidential candidate in 1980, said Harris and Parris both represent the "corporate interests." He said the Democrats too long have lulled progressive voters by protraying themselves as the lesser of two evils.

"They'll throw a few crumbs at the working people of America," Morrill said. "Herb Harris's programs will always be coalition, Band-Aid approaches that are doomed from the beginning."