For more than a decade, Herbert E. Harris II and Ira M. Lechner were liberal fixtures in Northern Virginia's conservative political landscape, railing against tax breaks for oil companies and championing the Equal Rights Amendment.

But on Tuesday, while Democrats in depressed areas of Virginia captured three Republican seats, some say Harris and Lechner were beaten by the prosperity of the Washington suburbs.

"I think it means the economy in this area just wasn't that bad," said Fairfax County Democratic Party Chairman Dottie Schick yesterday. Unemployment in the 8th and l0th congressional districts, where Rep. Stanford E. Parris and Frank R. Wolf won reelection, has remained about 4 percent, unlike pockets of Southside and coal country Virginia where it has soared to above 10 percent.

The failure of their campaigns, which hinged on attacking Reaganomics, dimmed the political futures of Harris, 56, a former three-term Democratic congressman, and Lechner, 48, a former delegate from Arlington who lost his third consecutive race in five years. The defeat was particularly heartbreaking for Harris, who lost a three-way race by 1 percent of the vote for the second election in a row.

"It would be very difficult for either one of them to come back," said Arlington Sheriff Jim Gondles, a Lechner supporter. "At what point do you stop asking people to work their guts out, give money, do tedious and boring jobs for somebody who just can't gain a majority?" he asked.

Wolf, 43, and Parris, 53, who squeaked into office two years ago in the Reagan victory, solidified their political bases by defeating two well-known and well-financed challengers. Neverthless, Democrats immediately began speculating about possible 1984 candidates, particularly in the 8th District, where Harris' commanding presence since 1974 has stifled other contenders.

"He's Parris been elected to Congress three times and at no time has he gotten above 50 percent," said State Sen. Richard L. Saslaw, one of those mentioned as a possible contender. "That hardly looks like someone who's in there forever."

Although Lechner said yesterday that he "ran a very successful strategy and wouldn't have done anything different," several of his advisers said Lechner's campaign underestimated Republican strength and relied too heavily on appeals to active and retired federal workers. Others said Lechner's decision to entirely eschew television advertising in favor of a last-minute, direct-mail blitz was also, in the words of one adviser, "very high risk" in the media-conscious suburbs.

"The Republican faithful is just as strong as the Democratic faithful in Northern Virginia and the federal employe vote is not monolithic," said Paul Goldman, one of Lechner's former campaign managers and a veteran Virginia political operative. "I just don't think that in Northern Virginia a very partisan Democratic appeal will work without a Watergate situation."

Several key Lechner advisers conceded that Wolf commanded a skilled political organization and ran a strong campaign in which he distanced himself from the Reagan administration and focused on local issues like National Airport and Turkey Run Farm.

"The lesson that Frank learned from Joel Broyhill is to keep your potholes fixed," said Fairfax party chairman Schick, referring to the district's former 11-term Republican congressman.

Lechner's campaign was based on his faulty projection of a record-low voter turnout in which he could have won with 75,000 votes. "We actually exceeded our projections," Lechner said. "The problem was that 170,000 people voted, instead of 150,000 . . . . The Republicans don't have any organization at all."

As to his plans, Lechner said: "How the hell would I know if I'm going to run again? The question is inappropriate."

Harris, who declined to concede on election night, awoke at 6 a.m. yesterday and began searching precinct tallies for the elusive 1,600 votes he needed. By afternoon, however, he had acknowledged that Parris had won their third match.

Harris declined to rule out a future political bid and blamed his loss on the 1.7 percent of the vote that went to Citizens Party candidate Austin W. Morrill Jr., on Parris' $700,000 compared to his own $400,000 campaign fund, and on Parris' sophisticated targeted mail operation. But Harris said he believed Reagan's last-minute television appeals were decisive. "I think they heard a president saying, 'I need more time, it's a tough job,' and that had a loyalty-type impact on a lot of voters," Harris said.

Parris attributed his victory to superior organization, to Virginia voter approval of some aspects of Reaganomics and to local issues. In particular, Parris said he was able to win Harris' home Mount Vernon district for the first time ever, in part because of a flap Parris created last week by releasing a letter from Democratic Gov. Charles S. Robb proposing to build a prison in southern Fairfax County.

In addition to Saslaw, Democrats mentioned as possible 1984 challengers to Parris included Fairfax Del. Vivian Watts, a big winner Tuesday; Fairfax Supervisor Sandra L. Duckworth; Fairfax Commonwealth's Attorney Robert F. Horan, who ran for Congress once before; Prince William Board Chairman Kathleen Seefeldt, and Prince William Del. David G. Brickley, also a big winner Tuesday.

Parris, pleased that for the first time he won without help at the top of the ticket, led Republican Sen.-elect Paul S. Trible throughout the district. Nonetheless, Parris said no one is ever invulnerable in volatile Northern Virginia. "That's the fatal fascination with Northern Virginia politics," Parris said yesterday before leaving for a goose hunting trip and a brief vacation in Europe. "There is no such thing as a sure thing."