Virginia Attorney General Gerald L. Baliles has advised the Fairfax Board of Supervisors that it is powerless to adopt voluntary limits on campaign contributions in an effort to curb the power land developers wield in local politics.

The supervisors adopted such guidelines for their past two elections and are considering similar limits next year, despite opposition from the business community and some party regulars. Baliles' opinion, while only advisory, is likely to strengthen the position of those who want to scuttle the guidelines altogether.

"I think we should ignore his opinion," said Supervisor Audrey Moore, a Democrat who supported Balilesin the November elections. "If you accepted his logic you couldn't drive 45 mph because you'd be voluntarily driving below 55, and that's ridiculous."

Moore's position was supported by the county Democratic committee, which voted Tuesday night to support the guidelines despite Baliles' opinion.

Other supervisors indicated the letter would influence their position. "I'd feel a little uncomfortable going directly against the attorney general's opinion," said Board Chairman John F. Herrity, a Republican who had favored the guidelines and said he would still follow them in his own campaign. "I don't believe you want to get into a lawsuit over the damn thing."

Under the voluntary guidelines, supervisors and many challengers agreed to disclose the names of anyone contributing $15 or more and to reject money from developers or cable television operators with business before the board. State law -- described by the president of the Fairfax League of Women Voters, Leslie Byrne, as "completely impotent" -- requires disclosure only of contributors of $101 or more.

Under state law, more than 90 percent of contributors to some 1979 supervisors' races would not have been disclosed.

Baliles' opinion came in response to a request from State Senate Majority Leader Hunter B. Andrews, an opponent of Fairfax's guidelines. Andrews, in turn, had been asked for the opinion by Supervisor Sandra L. Duckworth.

Baliles wrote that Fairfax, like other Virginia localities, cannot enact laws in any area without express permission from the General Assembly. He said that although Fairfax calls the guidelines voluntary, they have the effect of regulating an area that the state already regulates.

"It may not be a total effect, and it may not be a totally effective effect, but it had some effect on regulating the election," a spokesman for Baliles said yesterday.

Supervisor James M. Scott, a longtime supporter of the guidelines, said candidates should pledge to limit their contributions even if the board takes no official action. He also questioned Baliles' interpretation of the guidelines, which one supervisor and several challengers declined to follow in 1979.