Strong local opposition to Los Angeles' court-ordered school busing program is threatening the reelection chances of Rep. James Corman (D-Calif.), a 20-year House veteran and chairman of the Democratic Congressional Campaign Committee.

Corman, who usually wins in this predominantly white, heavily Democratic middle-class San Fernando Valley district with at least 60 percent of the vote, is facing his toughest challenge in a decade from Republican Bobbi Fiedler, a member of the Los Angeles Board of Education and an anti-busing leader.

"It's the only issue she has in this campaign," Corman, 60, said. "If we discussed anything else she couldn't make a case for her election. This is a good Democratic district. It's the only reason it's close."

Fiedler, 43, was elected to the school board -- her first public office -- in 1977 from the valley, a suburban area and a hotbed of anti-busing sentiment. As leader of an organization called Bus Stop, she has spearheaded opposition to the school desegregation order issued by State Superior Court Judge Paul Egly.

The Fiedler-led anti-busing faction, which won control of the school board last year, has raised new legal challenges to the court-ordered desegregation plan. That plan calls for reassessing 80,000 of the city's 540,000 public school students this year.

As school opened Tuesday, the board only bused some 25,000 students, hoping for a state supreme court order to allow them to scale down Egly's plan.

Wednesday, however, the board lost its appeal at the state supreme court and now must comply with the entire plan by Sept. 29.

Fiedler insists that Corman's open espousal of school busing and his refusal to support anti-busing amendments in Congress is only the most glaring example of his insensitivity to his district. Strategists on both sides of the campaign agree that over 80 percent of voters in the district oppost mandatory school busing.

"Corman doesn't represent the San Fernando Valley anymore; he represents the liberal bureaucracy, Washington, Jimmy Carter and [House Speaker] Tip O'Neill," Fiedler said. "Busing is just one example. He's so out of touch he doesn't know what people in the district think."

In recent weeks Fiedler has expanded her attacks on Corman to include other issues, particularly his support for welfare programs and tax bills opposed by business interests during his years on the Ways and Means Committee. Fiedler, who is Jewish, also has criticized Corman's support for what she describes as "the pro-Arab" Middle East policy of the Carter administration, as well as his opposition to construction of the B1 bomber.

"He is a very conventional hard-core liberal, with Carter all the way down the line," said Fiedler's chief campaign strategist, Arnie Steinberg, a one-time aide to former New York senator James Buckley. "He lives in another world. He lives in Lyndon Johnson's Great Society."

Both sides concede that Fiedler must cut deeply into Corman's traditionally heavy support among labor union members and Jews, who make up between 15 and 20 percent of the district's electorate. More than 60 percent of the district's registered voters are Democrats.

Corman's strategists see their task as keeping the congressman's basic constituencies intact despite the busing issue. Corman, in the face of much political pressure, has refused to denounce the concept of mandatory busing, although he has been critical of some aspects of the court-ordered plan.

Clint Riley, Corman's campaign manager, believes the congressman's long record in support of tax breaks for working people, national health insurance, and aid to Israel will outweigh voters' concern over busing.

Corman's literature also stressed the congressman's role in the helping to bring more than $1 billion in military contracts and tens of thousands of jobs into the valley last year.

"The key to this election," one Democratic strategist said, "is if people have to put food on their tables, or as parents who don't want to send their kids on a school bus every day. That's the contest that's really going on here."

Strategists on both sides agree that the race is still too close to call. Steinberg claims that the challenger is within 5 to 10 percent of Corman among voters, while Corman strategists insist their candidate is further ahead.

Both sides are spending heavily in this campaign. Corman plans to spend up to $400,000, much of it from labor unions. This would be more than 10 times what he used in his 1978 campaign. Fiedler, with the active backing of the U.S. Chamber of Commerce and other business political action committees, expects to gather a war chest of more than $350,000.

Despite discussion of national issues, and the funds coming in from all over the country, busing is likely to prove the determining issue in the contest. Fiedler, as a member of the Board of Education, is in the awkward position over the next few weeks of having to implement the mandatory program she opposes with the minimum of chaos.