John F. Herrity, the balding, acerbic Republican chairman of the Fairfax Board of Supervisors, is a frustrating enigma to his Democratic adversaries.

They ridicule his brash style, call him a blusterer and question his depth of knowledge on issues. Yet they readily acknowledge that the 51-year-old insurance salesman, a member of the county board for 12 years and its chairman for eight years, is extremely popular in the county.

This fall, the candidate to take up the Democratic gauntlet against Herrity is Pat Watt, 44, an executive with a Washington association who has never run for public office. Four years ago, she managed the Democratic campaign of Vivian Watts, whom Herrity crushed, winning 56 percent of the vote to Watts' 44 percent.

Watt acknowledges that the odds again favor Herrity. But she predicts she will upset Herrity on Nov. 8, the day Virginia voters will elect many local officials and fill all 140 seats in the state legislature.

"I am certainly an underdog, but I smell a chance to pull it off," says Watt, a serious, tiny-boned woman whose soft voice carries traces of a burr from her native Scotland.

Labor Day marks the kickoff of the campaign, and most candidates in Northern Virginia already have begun maneuvering to set their campaigns into high gear for the remaining nine weeks.

Thus far the struggle over the chairmanship of the Fairfax board, which sets governmental policies in the wealthy, fast-growing county, has been one of contrasts.

Watt, a chemist, was born in a Scottish fishing village and came to America 21 years ago with her husband, a chemist she met in England when he knocked over a lab experiment she was working on. She became a U.S. citizen in 1968 and has lived in Fairfax with her family since 1971.

Herrity was born in Arlington and has spent most of his life in the Washington area. He was trained as a lawyer, but supplements his $21,589-a-year supervisor's salary by selling insurance. He has aligned himself with the county's conservative forces, speaking out against various public housing projects and for better roads and lower taxes.

Herrity is unabashed, whether he is bumming cigarettes, jogging down Fairfax streets or seizing on issues such as the Lorton prison, the library system's R-rated video cassettes and the plight of the Falls Church High School marching band, whose wool uniforms were ruined in the downpour at the Redskins Super Bowl victory parade.

When he lost a bitter congressional race to Democratic Rep. Herbert E. Harris in 1978, Herrity said his defeat was "a message" from some voters that they wanted him to remain chairman of the board, the county's top elected position.

Although the job makes Herrity the equal of D.C. Mayor Marion Barry or Prince George's County Executive Parris Glendening, the Fairfax position is part time, and Herrity's duties are largely ceremonial. The day-to-day administration of Fairfax's $1 billion budget is handled by an appointed county executive, a fact that some say often frustrates Herrity's ventures into policy-making.

He has never been at a loss for words. When Watt announced her candidacy, Herrity bounded into the county press room in the Massey Building, asking: "Hey, did you hear that Pat Watt is Jim Watt's sister?" When told that the Democrat wasn't related to the controversial Interior Department secretary, Herrity shrugged, laughed and repeated his tale to the next reporter.

By comparison, Watt seems shy. When local and state officials gathered in Fairfax this summer to thrash out transportation-funding politics, Watt, the only challenger in attendance, hung to the back of the room. "Is this where the observers sit?" she asked, glancing around the room.

A former League of Women Voters president, she got her start in civic affairs when she became chairman of the Horizon's Day Camp, where about 50 black and 50 white children met for two weeks before school started to ease the tensions of school integration. Watt said the job reflected the needs of her family, which includes her black daughter Karley, whom she and her husband adopted in 1968 after having two sons.

Watt is running her campaign out of her Annandale home in the Camelot subdivision and is dependent on a network of volunteers. Because of that, she says she will be able to conduct her campaign on a $50,000 budget. She says she has raised about half that amount.

Herrity, who has operated a campaign headquarters in a Springfield storefront for about seven months, says he has raised roughly $30,000. He spent about $34,000 in his race four years ago.

During off years, Herrity averages about two community meetings a day, and he says he has stepped up his schedule this year. He is also, according to other Republicans, trying to take the "Rose Garden" approach to politicking. When asked by reporters what he thinks of Watt, he consistently replies: "I have no comment on my opponent."

Meanwhile Watt, who took a leave from her job with the Municipal Finance Officers Association, has been trying to battle Herrity's better name recognition by going door to door. She says she has knocked on about 4,500 doors throughout the county and hopes to go to another 4,000 in the next two months. The chairmanship is the only at-large seat on the nine-member Fairfax board.

Watt boasts endorsements from the Fairfax Education Association, the American Federation of State, County and Municipal Employees, and Local 400 of the AFL-CIO United Food and Commercial Workers Union. Herrity says that he doesn't believe in endorsements.

Watt says that if elected chairman, she would increase the county transportation department staff and would appoint a transportation task force to find a solution to the area's traffic problems. She says that Herrity promised to set up a transportation task force eight years ago but never followed through.

"My voting record is there," Herrity said. "I don't think the people in this county are looking for another study, they're looking for solutions."

Watt also sharply criticized Herrity for supporting the 3 percent cap on teachers' salaries last summer. Herrity shrugged at that charge, saying, "If you want to raise taxes, that's fine."

Both transportation and education are expected to be key issues in the other Northern Virginia campaigns. In a hotly contested legislative race, Democrat Emilie Miller and Fairfax City Mayor John W. Russell, a Republican, are facing off for the state Senate seat vacated by Adelard L. (Abe) Brault, dean of the Northern Virginia legislative delegation.

Del. Nora A. Squyres, a Democrat and advocate of the Equal Rights Amendment, and former delegate Gwendalyn F. Cody, a Republican and outspoken opponent of the ERA and abortion, are pitted against each other in a rematch of the 1982 battle in which Squyres unseated the conservative Cody with the help of women's groups.

In what is predicted to be one of the closest of the eight Fairfax supervisor races, Democratic incumbent Sandra L. Duckworth faces Republican Carol Ann Coryell for the Mount Vernon seat. And Democratic Supervisor James M. Scott is challenged by Republican Stephen A. Armstrong and independent William M. Lockwood for the Providence District seat.

Democrat Philip L. Chabot, an Alexandria lawyer, hopes to unseat incumbent Republican Del. Frank Medico, who represents the 44th House District in another Mount Vernon-area race that is expected to be close.

In Arlington, two seats on the five-member county board will be up for election with Democrats Richard A. Buffum and Albert C. Eisenberg seeking to oust GOP incumbent Walter L. Frankland Jr., who is seeking his third term, and his running mate Michael E. Brunner, a former member of the county school board. Board member Dorothy S. Grotos, who has been a popular GOP vote-getter, is seeking the full-time position of county treasurer there and faces Democrat Frank O'Leary.

Loudoun County voters will select a successor to controversial Republican sheriff Don Lacy, who did not seek reelection. A special grand jury recommended that he resign because of his private conduct and alleged personal abuses of power. Candidates are John R. Isom, a captain in the Fairfax Police Department, and Republican Leonard W. McDonald.

Prince William voters will choose between Republican Harry Hittle and Democrat Wilson Carlin Garrison Jr. to replace retiring Sheriff Carl A. Rollins Jr.

A few candidates are running unopposed. Among them are: Fairfax Democratic supervisors Martha V. Pennino and Audrey Moore; Fairfax Sheriff M. Wayne Huggins, a Republican; Fairfax prosecutor Robert F. Horan, a Democrat; Del. Warren G. Stambaugh (D-Arlington); and state Sen. Charles J. Colgan (D-Prince William).

An article in yesterday's editions incorrectly identified the campaign managed by Pat Watt, Democratic candidate for Fairfax County Board chairman. She managed Vivian Watts' successful race for the Virginia House of Delegates in 1981.