Former Virginia legislator Ira M. Lechner trounced two challengers in Northern Virginia's Democratic primary yesterday, winning the nomination to oppose freshman Republican Rep. Frank R. Wolf in November.

Complete, but unofficial returns showed Lechner, a 48-year-old labor lawyer from Arlington, won with about 84 percent of the vote in an extremely light turnout. His closest challenger, Rose Z. Thorman, 56, of Arlington, received 10 percent, while McLean media consultant Edward D. (Ted) McLaughlin Jr., 51, trailed with 6 percent. Only 5.5 percent of the 10th Congressional District's 255,000 registered voters participated in the primary.

Because he enjoyed name recognition and a superior organization, Lechner, who had lost two consecutive races for lieutenant governor, had been favored to win the primary, the only congressional primary in the Washington area.

"We took every precinct in the 10th," a jubilant Lechner said last night. "It shows the Democratic Party is completely unified for November and I think it's a personal vote of confidence in me. The whole thing signifies a resounding Democratic victory in November."

Thorman, a former Interior Department official who called 1982 "the year of the woman candidate" picked up some support from party regulars who resented Lechner's often brash style. "We did pretty well," said Thorman last night as she pledged to support Lechner. "I said all along either of my competitors is certainly more qualified than Frank Wolf."

Party leaders had discounted the candidacy of McLaughlin, who in turn said he "didn't think much of the Democratic organization here." McLaughlin accepted his defeat last night, saying, "Not as many people turned out as I thought they would."

The three candidates were virtually indistinguishable on the issues. All supported a freeze on nuclear weapons and opposed the Reagan administration budget cuts that have cost federal workers their jobs. That is a particularly sensitive issue in the congressional district that has more active and retired federal workers than any other in the country.

As the Democratic nominee, Lechner, who is vice chairman of the state party, plans to make the fall race a referendum on the economic policies Wolf has supported.

Wolf, 43, says he is proud of his record and wants to prevent a "well-organized and well-financed" Democrat from returning the federal government "to the way it was in the 1960s and 70s."

The race has been targeted by affiliates of both national parties. The Democrats, who regard Wolf as vulnerable, are trying to recapture the seat they lost two years ago when the Vienna lawyer-lobbyist rode to victory on Reagan's coattails, defeating three-term incumbent Joseph L. Fisher. Last December Fisher disappointed many Democrats by deciding not to run for his old seat, accepting instead a job as Virginia's secretary of Human Resources.

Lechner, who is regarded by strategists in both parties as a tireless campaigner and skilled debater, says that raising money for the campaign will be one of his chief priorities. This summer he plans to hold a series of out-of-state fund-raisers in a dozen cities with large Democratic populations, including Los Angeles, Boston, New York and Atlanta.

So far he has raised about $65,000, much of it from labor unions. Wolf has raised twice that amount, most of it from corporate political action committees, the same sources he tapped two years ago as a challenger when he waged one of the most expensive congressional campaigns in Virginia history.

In addition to raising money, party workers say Lechner must try to moderate his image as a liberal loser in a district where the GOP has made substantial gains. Lechner minimizes his 1977 and 1981 defeats for the lieutenant governorship, saying he has always carried the 10th District, which includes Fairfax City, Falls Church, Arlington and Loudoun counties and the northern half of Fairfax County.

Although Lechner attracts a cadre of loyal campaign workers who admire his outspoken advocacy of progressive causes, his aggressive tactics have alienated some party leaders in Fairfax County, the most populous jurisdiction in the district.

Some of them, including State Sen. Adelard L. (Abe) Brault, the dean of the Northern Virginia delegation, supported Thorman in the primary partly as a protest of what one of them called "Ira's street-fighting techniques." Others, who remained neutral in the primary, said party unity is more important than personal differences and have pledged to support Lechner.