Even during the voice checks, before their first debate was on the air, the candidates were sparring as only old enemies can.

"This is Herb Harris from Mount Vernon," the Democrat said, and then -- with a wink to the man in the studio -- "your future congressman from Virginia."

"I'm Stan Parris," the Republican responded, unsmiling. "I am the congressman and I expect to stay that way."

Herbert E. Harris II, Northern Virginia's spirited former congressman, and Stanford E. Parris, the district's silver-haired Republican incumbent, met this week in the first debates of this fall's campaign, at WMAL radio Wednesday night and before the Northern Virginia Press Club yesterday. The debates proved that the candidates, facing each other for their third time in their careers, have not grown friendlier with time.

Harris exuberantly goaded his opponent in an effort to overcome the Republican's lead in the polls. Parris, literally biting his lips at times, ignored some of Harris's barbs but could not resist responding to others.

Both candidates focused on the economy and, specifically, on whether current economic problems should be blamed on three decades of Democratic Congresses or two years of Reaganomics.

"I remember what the '30s were like, and I don't want to see that again," Harris, 56, said yesterday. "It's awfully hard for Parris to explain to people who have worked all their lives for a safe and secure retirement, to explain how you vote to freeze their annuities, to cut their medical benefits and to say, 'We don't remember, we don't appreciate, this is the new America.'"

"To hear Herb Harris talk about deficits is like seeing Jack the Ripper go to the hospital to bandage up his victims," Parris said during the Wednesday radio debate. "Guess who got us to the place where we've got a deficit of $150 billion by voting for every crackpot, socialistic scheme that the Congress ever dreamed up for the six years he was in Congress?"

Parris was elected congressman in 1972 and was turned out of office by Harris in the Watergate landslide two years later. Harris served six years, representing Alexandria, southern Fairfax County and parts of Prince William and Stafford counties, and then was bounced by Parris in the 1980 Reagan landslide.

Now the two former Fairfax County supervisors are squaring off again, joined this time by Citizens Party candidate Austin W. Morrill Jr. Morrill, a 33-year-old social counselor, is pushing a nuclear weapons freeze, more emphasis on human rights in foreign policy and development of nonnuclear, nonpetroleum energy. "Read Barry Commoner's books," he soberly urged his audience yesterday. "He has some very exciting ideas."

Harris and Parris were reluctant to depart from economic issues during the debates, although they revealed significant differences when they did. Without taking a stand on proposals now before the Senate, Parris said he opposes abortion except in cases of rape or incest and Harris said he would oppose the constitutional amendments now pending on Capitol Hill that would forbid abortions.

Parris said he supports nuclear energy as "the most environmentally benign, the cheapest and the safest we have," while Harris said he stands on "middle ground" with respect to nuclear energy and believes Virginia Electric and Power Co. has relied too heavily on nuclear plants. Harris said the policies of Interior Secretary James Watt are a "disgrace," while Parris deflected questions about Watt to a discussion of the progress he has made resolving Alexandria's dispute with the federal government over who owns the city's riverfront.

Only when discussing deficits and Social Security and federal employe pay rises, however, did the two men get excited, with Harris calling his opponent "Stanford" like an impatient father using his child's full name. Harris managed to draw the congressman, somewhat to his staff's dismay, into lengthy discussions Wednesday on why he had failed to appear at a forum of retired federal employes and just which "socialistic schemes" Parris had in mind when he attacked his opponent. (Parris would name only the Tennessee-Tombigbee Waterway, which Harris said he opposes.)

Parris and his staff replayed the Wednesday debate about 40 times, according to one aide, and yesterday the Republican for the most part stuck to his strategy of remaining congressional and aloof. "I'm not here to reply to your questions, Herb," Parris said during one of Harris' attacks.

Still, neither candidate is likely to remain too aloof. When Harris held a six-mile "Run for Congress" fund-raiser on Labor Day, a Parris aide persuaded a marathon runner from the Marine Barracks in Washington to enter the race and don a Parris T-shirt as he approached the finish line.

Unfortunately for Parris, the aide said, the runner finished fourth.

Not so, a Harris spokesman insisted yesterday; he said the Parris runner finished sixth.