Former Virginia governor Charles S. Robb has endorsed Fairfax County Supervisor Audrey Moore of Annandale, the Democratic candidate for county board chairman, and said he would appear in a television commercial on her behalf, Moore's campaign announced yesterday.

Democrats said support from fellow Democrat Robb, a Fairfax resident who is popular with corporate leaders in the county, was a blow to Republicans and may help to temper business opposition to Moore's candidacy.

Republicans generally shrugged off the announcement as meaningless, citing other local races in which endorsements by prominent leaders, including Robb, failed to provide victory for Democrats.

Moore is challenging Republican Chairman John F. Herrity for the county's top elected post. The election is Nov. 3.

Robb is generally credited with having shifted coalition politics in Virginia by winning over many business leaders through aggressive economic development policies.

By contrast, Moore has made her reputation in 15 years on the county board as a leading opponent of rapid growth, a position that many business people have regarded as hostile to their interests.

Thus, Robb's endorsement of Moore -- particularly his decision to appear in a television commercial on her behalf -- is seen by her supporters as lending credence to her frequent assertions that she is not against economic growth, as many assert, but simply in favor of more rigorous planning.

In a brief statement released by his office, Robb said that Moore "has assured me that if she is elected chairman she will work closely with the business leadership to continue to move Fairfax County forward."

Robb's statement also noted that Moore, who has frequently been on the short end of 8-to-1 votes on the board, has "demonstrated the courage of her convictions." He said he was "pleased to support her candidacy."

Herrity, chairman of the board since 1976, is not short of big-name backers himself. Senate Minority Leader Robert J. Dole, a presidential hopeful, and his wife, Transportation Secretary Elizabeth Dole, are holding a $100-a-plate breakfast fund-raiser for Herrity tomorrow at the McLean Hilton. The event, Herrity's largest fund-raiser of the fall, is expected to raise at least $70,000 for his campaign for a fourth term.