Defying a ritual of local politics, two of Fairfax County's most successful vote-getters are distancing themselves from their party's nominees in the race for chairman of the County Board of Supervisors.

On the Democratic side, Supervisor Joseph Alexander of the Lee District, a conservative who has held office since 1964, has so far declined to endorse Supervisor Audrey Moore, the board's leading advocate of slowing the pace of development in the county.

On the Republican side, Supervisor Thomas M. Davis III of the Mason District, a popular moderate whose votes on growth recently have coincided with Moore's, has endorsed Republican Board Chairman John F. Herrity -- but with a distinct lack of enthusiasm.

The reluctance of Alexander and Davis to embrace their party's nominees is bad news for Moore and Herrity and underscores the trepidation with which some officials are approaching this year's high-stakes race for the chairmanship.

Alexander and Davis, who probably will be unopposed in their home districts this year, are extremely popular, and many analysts think they could sway votes for Moore and Herrity if they wanted.

Yet to the dismay of aides to Moore and Herrity, neither man seems inclined to play an active role in the race for the chairmanship.

In an interview, Davis said he "works very well with" Moore. Asked about Herrity's performance, Davis was silent for nearly 30 seconds before answering that Herrity had done a "good job" of presiding over the economic boom -- "some would argue too good a job."

Davis has scheduled two weeks of vacation out of town in the fall -- just when Herrity would most need his help.

Four years ago, Davis collected more votes than any candidate in the county in crushing his Democratic opponent by a 3-to-1 ratio. Much of his support came from moderate Democrats. With the help of Davis' strong showing, the district also voted heavily for Herrity.

This year, things have changed. Since the last election, Herrity has opposed several initiatives that were close to Davis' heart, including an ordinance to limit smoking in restaurants, a $30 million neighborhood improvement bond in 1985 and a proposal last winter to restrict office construction on about 10,000 acres around the county.

In addition, many Republicans say, Davis appears to think that Herrity, who was convicted last year on a misdemeanor charge of violating a state disclosure law, is in political trouble this year. Polls show Moore with an early lead, and Davis has angered Herrity campaign officials by saying that Herrity is trailing in the Mason District.

Davis also knows that Herrity suffers from a public perception that he is closely aligned with developers -- the local equivalent of being soft on communism.

Although some Republicans say Davis is simply blowing with the prevailing political winds, Davis insists that he has not abandoned party loyalty and that he does stand behind Herrity. "He's the Republican nominee, for heaven's sake," said Davis. But he added: "I happen to share a common border with Mrs. Moore's district and we have a good working relationship."

For Alexander, the political situation is even more sticky. It is an open secret among county officials, board members and journalists that Alexander, who is strongly progrowth, does not much like Moore or the positions she espouses. Conversely, he enjoys a friendly, at times jocular, relationship with Herrity, who frequently refers to Alexander with mock reverence as "the oldest surviving member of the board."

Moreover, Alexander and Herrity have been on the same side of the growth issue, each stressing the benefits of the county's economic success, such as lower tax rates and improved services.

Recently Alexander has answered "nope" when asked if he has decided what role to play in the race for chairman. Said a county official: "Joe's lying low . . . . He knows a no-win situation when he sees one."

Democrats say it is unlikely that Alexander will jump party ranks and endorse Herrity, although he has endorsed Republican candidates for statewide office in the past.

But by withholding from Moore what is usually a routine endorsement, he also deprives her of all-important name-dropping privileges in Lee District, an area that some strategists say is up for grabs in the chairman's race.