Marie Ridder of McLean has dropped plans to challenge Democrat Ira Lechner in his bid for the Northern Virginia congressional seat now held by freshman Republican Frank R. Wolf, a move that eliminates the most serious challenge to Lechner's candidacy.

"Even though I personally don't think he Lechner is the best candidate, I am not going to run because I think the cost of the primary is immoral," said Ridder, a free-lance journalist who is married to Knight-Ridder newspaper executive Walter Ridder. "It's just wrong to raise that much money--$250,000--in a primary against another Democrat when you've got to raise more for a general election."

"Ira is the person to beat Frank Wolf," said Paul Goldman, Lechner's campaign manager. Goldman agreed Ridder's withdrawal eliminates one of the most formidable challenges to the Arlington Democrat's candidacy.

Although she had not formally announced her candidacy, Ridder, at the urging of friends including Rep. Morris Udall (D-Ariz.), has spent the past several months raising money and taking polls in the 10th District, which includes Arlington and Loudoun counties and the northern half of Fairfax County.

Some Democratic strategists regarded her as a threat to Lechner's candidacy because of her personal wealth, her ability to raise funds and because she is not tarred with the liberal brush that has hurt Lechner in the past.

Others say Ridder lacked three crucial ingredients: name recognition, recent involvement in the local Democratic Party and a strong organization.

"I think one of Marie's problems was that she had to call up and introduce herself to people, whereas Ira has instant name recognition among the active folks," said one who had considered running for Wolf's seat.

A former member of the Virginia House of Delegates, Lechner, an attorney, is the primary architect of the state's 1974 Tenant-Landlord Act. He espouses a brand of old-fashioned liberalism that some fear could hurt the Democrats in Loudoun and Fairfax counties, where Republicans have made substantial inroads recently.

Lechner's only announced challenger in the June 8 Democratic primary is Ted McLaughlin of McLean, president of a Northern Virginia consulting firm and a former Department of Housing and Urban Development official making his first political bid.

McLaughlin, an unknown among party regulars, is campaigning on the experience he acquired during his 15 years as a federal employe. He says he will promote a "true, balanced economic program" and advocates restoring some of the cuts in social programs made by the Reagan administration.

Rose Thorman, 55, of Arlington, says she also is considering challenging Lechner. The former manager of federal women's programs at the Bureau of Mines, Thorman says her chief strengths are her familiarity with Congress and her feeling that she could beat Wolf more easily than Lechner could.

Ridder, meanwhile, says she is considering running for the delegate seat now held by Republican Robert Andrews. "I'd like to be a better politician before I make a definite decision on that," she said.