Fairfax County board chairman John F. Herrity, who is seeking an expanded leadership role, was rebuffed yesterday in his bid to represent the Northern Virginia suburbs on one of the area's key regional authorities.

Apparently lacking the votes for a board nomination to the areawide Council of Governments, Herrity withdrew himself from consideration, citing a need for "as a great a degree of harmony as possible" among board members.

Instead, the board named Supervisor Martha Pennino (D-Centreville) to the position. Pennino reelected yesterday to her ninth year as board vice chairman, is increasingly seen as Herrity's main challenger on the Fairfax board, leading board moderates in what she calls a "philosophical" battle against Republican Herrity's brand of conservatism.

Also named as the county's representative to COG was freshman Supervisor Nancy Falck (D-Dranesville), who was offered by Herrity as a compromise candidate.

Long-time board watchers saw the action as the board's effort to send Herrity a message that it wants conciliatory, statesmanlike leadership rather than Herrity's blustery, combative style. "We didn't want that kind of image throughout the region," said one former top county official.

Some board members also said privately that they feared an appointment to the COG post would give the 47-year-old Herrity the opportunity to promote his own political fortunes rather than the interests of the county he was supposed to represent.

"People were afraid he might use it as a stepping stone to a congressional campaign," said Supervsior Audrey Moore (D-Annandale) who said she opposed Herrity's bid for the post.

Herrity said yesterday, "I don't feel threatened by anything that happened in the appointment process today."

Herrity, who began his second term on the Fairfax Board this month, lost a congressional race in 1978 to Rep. Herbert E. Harris (D-Va.).

Shortly after his re-election last November, Herrity, who has been called a demagogue and worse by his colleagues, said he would tone down his behavior and become "less visible and more dominant."

COG is a regional planning body that coordinates programs on water, transportation, public safety, health and human resources. It has been gaining in prominence since the federal government in recent years began to tie regional planning to the allocation of million of dollars in federal funds.

Herrity's decision to stand aside would seem to indicate that his ambition of becoming a "consensus builder" on the board has not yet been realized.

Herrity was expected to gain power when Republicans picked up an additional seat on the Fairfax board in last November's election, bringing their numbers to four of the board's nine members.

Pennino denied yesterday that she was leading a Democratic challenge against the board's Republican chairman, saying only that she expected to see continual duels between the board's moderates and conservatives over the next four years.

The COG appointments could signal the beginning of a stormy political period for the Fairfax board. Two subsequent votes -- over repealing the county's bottle ordinance and instituting a county auditor -- also found Pennino and Herrity at odds yesterday.

Herrity was the victor in each of those contests, leading a 5-4 majority in favor of the motions. Voting with him were veteran board members Joseph Alexander (D-Lee) with newly elected supervisors Falck, Sandra L. Duckworth (R-Mt. Vernon) and Thomas M. Davis (R-Mason).

In Pennino's camp were Moore, James M. Scott (D-Providence) and Republican Marie B. Travesky of Springfield.