Fairfax County Board Chairman John F. Herrity, painting a picture of stability and prosperity in the county he has led for 12 years, declared yesterday that there is no reason he should not be reelected to a fourth term in office.

In an interview with editors and reporters of The Washington Post, Herrity also cited a rising threat to the county posed by national and local economic trends -- the tumult on Wall Street, rising interest rates and climbing office vacancy rates -- and warned that his Democratic challenger, Supervisor Audrey Moore of the Annandale District, would bring an antibusiness climate to Fairfax when the county can least afford it.

"Why should I be replaced," asked Herrity, a Republican, "when the county is in outstanding prosperity, has the highest level of services of any county in the country, when the tax rate is down by 22 percent since I took office as chairman? Why is there a need to replace me?"

Herrity, 55, is facing his toughest reelection fight. Moore, 58, who has served on the county board since 1972, is running on a platform of slowing the county's growth -- a message that has dominated her political career. The race, by far the most expensive for a local office in Washington's suburbs, will culminate on Election Day, Nov. 3.

A Washington Post poll published yesterday showed that Herrity has gained ground rapidly in recent weeks in his race against Moore, who had been ahead by a substantial margin last month.

Herrity said yesterday that while he does not put a lot of stock in polling results, he was heartened by the trends and expects his campaign's fund raising and morale to perk up as a result.

Herrity has apparently benefited in recent weeks from a television and radio campaign that attacks Moore while barely mentioning him. He said yesterday that he decided on that strategy when it became apparent that Moore's record was not well known and that the media were not focusing on how she has voted.

"The public hasn't known Audrey Moore," he said. "I think they're beginning to know her . . . . We had to focus in on her record there."

He said the commercials attacking Moore's record are one of "a number of focuses" in his campaign. "I run on my record," he said.

Herrity contrasted what he called his proven ability to lead the county and to work for regional solutions with Moore's voting record and slow-growth program, which he described as "unstable."

Although Herrity refrained from the personal attacks he has made on Moore in the past, he assailed her slow-growth policies as a prescription for disaster at a time of economic uncertainty.

"Prosperity's not necessarily forever," he said. "Once you destroy the confidence of the business community and the stability and the integrity of the government, I can assure you, you have some serious problems."

"With the economy as shaky as it is, we could have some real problems," he said. "I don't think Mrs. Moore can work with a unified board or work regionally."

Herrity said that if Moore were elected, her attempts to slow the pace of growth would be overturned by Virginia's courts and legislature. "But in the short term, it's going to be rather disastrous," he added. He called an attempt in the early 1970s to slow growth in Fairfax "an absolute disgrace and an absolute mess."

He said Moore's efforts to control the pace of growth by opposing certain proposals for new roads and public facilities were "an anchor around my neck."

Herrity, who has presided over a period of unrivaled growth and economic development, noted Fairfax's success in attracting leading corporations from around the nation to establish headquarters and offices.

But he warned that those successes could be ephemeral if the county government takes an antibusiness turn.

If Moore is elected, Herrity said, "there are other counties and other jurisdictions -- Prince George's, Prince William, Loudoun -- . . . that would love all those nice corporations that have landed in Fairfax County and they'd love to lure them out of Fairfax."

Herrity stuck by his long-held position that there is no need for major zoning or land-use changes that would restrict the amount of commercial development permitted under current county regulations.

Moore has supported such changes as a method of bringing road construction up to speed with development.

When asked if he would support any major shifts in Fairfax's development policies, he declined to be specific, but he did say that a county commission is studying the issue.

The commission is not scheduled to make its recommendations until after the election.

The Post's poll has shown that three-quarters of county voters want to slow the pace of development.

A plurality of 42 percent says the cause of the county's traffic problems is overdevelopment -- a perception that the county's business community has sought to dispel.

A year ago, the Fairfax board rejected on a 5-to-4 vote a proposal that would have severely curtailed the amount of office construction permitted on about 10,000 acres around the county.

Moore voted for the proposal, which was designed to address traffic snarls; Herrity sided with developers and business leaders in opposing it, arguing that it would undercut the county's tax base.

Yesterday, he described the effect of such a shift in dire terms.

"Companies' whole faith in Fairfax County would have been destroyed," he said. "I'm not sure that by crippling the economy, you solve transportation problems."

The county board chairmanship pays the same annual salary as the eight district supervisors' seats: $21,589. That is scheduled to jump to $35,000 next year and, incrementally, to $45,000 by 1991.

As he has in the past, Herrity portrayed his job as chairman of the nine-member Board of Supervisors as one with "no inherent powers except to run meetings."

He said, "The job as chairman is about as powerful as the {incumbent's} ability to run things and move the board and work for regional agreements and accomplish things . . . . I've made the job what it is because of my leadership."

At the same time, however, he bemoaned the idea that he is to blame for transportation problems, noting that road-building in Virginia had been exclusively a state responsibility until recently.

Polls have shown that a large majority of Fairfax residents consider transportation the county's number one problem, and Herrity and Moore have taken turns throughout the campaign blaming each other for the traffic tie-ups that dominate the roadways.

Herrity reiterated that theme yesterday, excoriating Moore for casting several votes against certain aspects of key road construction projects. Moore has maintained that she supported the projects generally and quibbled only with the specifics.

On the question of the shortage of low-cost housing in Fairfax, often mentioned as one of the county's most pressing needs, Herrity reiterated a long-held position, saying, "Housing is one of the things we can do the least about, in my opinion."

Herrity stressed the importance of adequate day care facilities, accessible to homes and work places. He acknowledged that he has not always been an advocate of day care services but changed his position when his son Tom, who is managing his campaign this year, "sat me down and started talking about it three or four years ago."

Fairfax has one of the highest proportions of working women in the nation, and day care has become an increasingly important issue in the county.

Herrity also mentioned the rapidly expanding elderly population in the county, but did not make policy proposals in that area.