AUDREY MOORE has issued a serious financial warning that her colleagues and constituents alike in Fairfax County cannot dismiss. The county board chairman has looked at the money and spending numbers and concluded that Fairfax is not immune to budget pressures being felt elsewhere -- locally and statewide. Fairfax is staring at a projected decline in revenue that, if not addressed quickly, could produce a deficit in the budget year that begins in July 1991. And like Gov. Wilder, Mrs. Moore would rather wind up wrong and with extra money to spend later than let the government go into the red. Here as elsewhere the groans of beleaguered taxpayers are audible enough to blow tax increases out of the picture for any politician contemplating reelection.

A deficit could reach $65 million in fiscal 1992, Mrs. Moore warns, if county and school spending continue increasing at their current levels of 8 and 10 percent respectively while property tax and sales tax revenues decline. She proposes cutting 1 to 2 percent in the school and county budgets during this current fiscal year and holding spending increases in the following 12 months to 4 percent. Mrs. Moore notes that she will strongly oppose any increase in taxes to make up for declining revenue.

Political opponents, including Supervisor Thomas M. Davis III, a Republican who is considering challenging Mrs. Moore for chairman next year, are quick to blame the county's financial straits on what they portray as the high-spending policies of the Democratic board. Precisely what they would have cut and how deeply is not entirely, clear, however, particularly given the boom years and the resulting high level of county services that have been the hallmark of life for most residents of the county.

"I could be wrong about our revenues in 1992," Mrs. Moore says. "As a matter of fact, I hope I am wrong. I hope that revenues exceed projections and that the state and federal governments increase aid to the county. If so, the 'down payment on the deficit' fund can be used to restore funds for the schools and county in fiscal year 1992, or to reduce taxes."

Members of the county board and school board may find better ways to cut the budget than across-the-board decreases. But another round of spending increases is definitely not on.