THE HOTTEST political contests of all in Virginia this year are likely to focus on the local leadership of Fairfax County. Already, struggles are shaping up within each party in the campaign for chairman of the County Board of Supervisors. Other members of the board may be challenged as well.

Audrey Moore, who led Democratic candidates for the board last time by trouncing the incumbent, Republican John F. Herrity, in a bitter battle, will go for a second term. She won't be going alone, though. At least one other Democrat, land-use lawyer Michael S. Horwatt, who stands to receive heavy financial support from developers, is seeking support for the party nomination. Mrs. Moore's arch rival, Mr. Herrity, isn't taking the voters' last no to him for an answer either. He's geared for a comeback, even if his party may not be. Republican Supervisor Thomas M. Davis III, who says he's a better consensus-builder than either Mrs. Moore or Mr. Herrity, has been prepping for this campaign for years.

The overriding issue for all candidates will be Mrs. Moore's performance. She won four years ago with an appeal to voters angered by "G and G": growth and gridlock. Now her opponents sense disillusionment among the same voters. They cite her reversals on local income taxes, revenue bonds for the Fairfax County Parkway and "downzoning," and her attempt to set new limits on development. Her defense is that she inherited tough situations that will take more than one term to turn around. In addition, economic pressures not of her doing are taking a local toll.

Among the would-be challengers, Mr. Davis contends that Mrs. Moore's on-again, off-again style has hurt the county. Mr. Horwatt echoes much of the sentiment of businessmen who believe Mrs. Moore stands a good chance of being rejected at the polls. Mr. Herrity is counting on his experience and the voters' intense feelings about taxes and bond issues. He is well known in the county, particularly among Republicans, but whether he is accepted as the GOP's 1991 answer to Mrs. Moore is still to be proved.

Restless as Fairfax voters may be, savvy politicians won't underestimate the ability of Audrey Moore to come out swinging and again galvanize a majority. How the county fares in the regular legislative session in Richmond this winter could offer the first clues to her fate in the fall.