Joseph L. Fisher's decision to join the coming administration of Gov.-elect Charles S. Robb sent Democrats in his Northern Virginia district scrambling yesterday for a new candidate to run against Rep. Frank Wolf (R-Va.) next November.

Moving swiftly to fill the void, former Arlington state delegate Ira Lechner is expected today to become the first Democrat to announce his candidacy for the 10th District congressional seat, but few expect Lechner to win the nomination without a fight.

Within hours after Fisher's appointment as state secretary of human resources, Democrats in his home district--which includes Arlington, Loudoun and northern Fairfax counties and the cities of Fairfax and Falls Church--began dividing along philosophic and jurisdictional lines, ending hopes for the kind of unified front that might have prevailed behind the popular 67-year-old former congressman.

Fisher, a three-term congressman who was ousted by Wolf in 1980, had all but announced a decision to seek reelection to the House of Representatives when Robb nominated him for the state cabinet post. Without Fisher, the field of Democratic candidates is wide open, the party's 10th District chairman, Charlie Rinker, said yesterday.

Lechner was unavailable for comment, but close advisers said the former state legislator is intent on seeking the Democratic nomination. "He's earned it, he's the front-runner and he's going to win it," said Paul Goldman, a Democratic strategist allied with Lechner and the liberal wing of the party.

But for many Democrats in the district, Lechner is far from the ideal candidate. His liberal credentials do not sit well with the more moderate wing of the party, now considered dominant after Robb's victory last month. Also, some Fairfax Democrats are eager to promote a candidate of their own who would be able to hold on to the Fairfax districts lost by Fisher to Wolf in 1980.

"When you look at the numbers, Arlington is just not enough," said one former Fairfax legislator. "To win you have to run strongly in Fairfax and for that, you need to have a strong candidate--a moderate Democrat--from Fairfax."

Traditionally, Arlington has been considered the Democratic stronghold in the 10th District, even though the number of Fairfax voters has always been greater. In 1974, Fisher won the primary by winning big in Arlington, even though he ran third in Fairfax.

But Fisher's loss in 1980, although interpreted by some as simply a fallout from the Reagan landslide, has given new ammunition to some Fairfax Democrats who note that the congressman's edge in Arlington was not enough to offset the more conservative vote in the outer suburbs.

Another factor in the battle over the Democratic nomination will be Fisher himself, who yesterday refused to rule out some involvement in the scramble for his old seat.

Other Democrats in the district also considering a run for the nomination include Arlington County Board member John Milliken, Arlington Del. Warren Stambaugh, former Fairfax delegate Raymond Vickery Jr. and Fairfax Democrat John Schell. Fairfax County Supervisor Martha Pennino, who lost a four-way primary to Fisher in 1974, has also been mentioned, but she was not available for comment.

Rinker said the 10th District Democratic Committee probably will decide to proceed with a June primary instead of a convention. "There is a strong feeling, generally speaking, in Northern Virginia that the most open process is a primary," Rinker said. The committee will make its decision in either February or March, he said.

So far, Lechner is the only candidate ready to make a decision in the race for Congress. Others, surprised by Fisher's announcement, said they expect to make up their minds in the next few weeks.

In Arlington, Milliken has emerged as Lechner's strongest opponent. Now serving his first term on the County Board, Milliken, a former aide to Fisher, has long aspired to Congress and is considered by many to have the moderate credentials needed to offset Lechner. But he stressed yesterday that he will not run to target a Democratic opponent. "If I run, I would not be running against anybody but Frank Wolf," he said.

Fairfax Democrats, drawing a lesson from the 1974 primary, stressed the need for unity if a Fairfax candidate is to emerge victorious in a party primary. "It is apparent to anyone from Fairfax that for anyone from Fairfax to have a reasonable political chance, we have to have just one candidate so we don't split the vote," said Vickery. Vickery, who had already pledged his support to Fisher, said yesterday personal considerations will be the key factor in his decision whether to run.

For all Democrats, however, the critical issue in the 10th District next year is Wolf and, by extension, the Republican economic program that he has espoused. Several Democrats predicted that Wolf is already damaged by a growing backlash to Reaganomics.

But some also predicted that a divisive party primary in the 10th District next summer could prove harmful to the Democratic party statewide. Goldman, who served as adviser to the Robb campaign, noted that Lechner and other liberals had put aside philosophical differences this year to work for Democratic victories. "Now," said Goldman, "the shoe is on the other foot."