Fairfax County's Republican Party is on the verge of withdrawing from the county's Fair Campaign Practices Commission because some party members believe the commission is biased toward Democrats.

Acting on a resolution adopted at a county GOP convention last month, the party's executive committee this week directed its chairman, William Olsou, to notify the commission that Republicans no longer will participate in the panel.

Olson said yesterday that he will forward the notice after the full Fairfax GOP committee meets later this month to endorse the withdrawal. That vote is regarded as a formality.

A well-placed GOP member said the initial resolution calling for withdrawal appeared to stem from a belief that the fair practices commission "had turned into a tool of the Democratic Party rather than an arbiter of electoral disputes."

The main controversy, however, can be traced to a bitter state senatorial contest last fall between State Sen. Adelard L. Brault, the region's senior Democratic legislator, and his young Republican challenger, John Thoburn.

Leslie Byrne, an independent who chairs the campaign commission -- composed of two independents, two Republicans and two Democrats -- said some Republicans were miffed when the commission ruled that Thoburn had distorted Brault's position on the abortion question.

The commission, which had been unable to locate Thoburn, issued the finding in his absence the Sunday night before the Tuesday election. Brault had complained to the commission that afternoon because Thoburn supporters had begun passing out last-minute campaign literature in the parking lot of Catholic churches that falsely claimed Brault was not an abortion opponent.

"We felt the nature of the complaint was such that we should rule on it beofre the election," said Byrne of the commission's 5-to-0 vote (with one abstention) that Thoburn had violated the county's fair campaign practices code.

"We believe there should be campaign ethics," said Byrne, who acknowledged that there were people in both parties who had never seen the need for the commission. The panel "has been a voter service.We get material from both sides . . . we're not out to be punitive but informative."

Set up as an ad hoc body in 1975 by the county's Board of Supervisors, the commission has relied mainly on the good will of both Democratic and Republican parties, who have agreed to abide by a campaign code of ethics. Party members are appointed through their party organizations.

The commission has no enforcement powers, but its rulings are made public.