Democrats in Fairfax County reportedly have settled on a candidate to challenge three-term incumbent Board Chairman John F. Herrity, a Republican whose popularity until now had scared off most potential challengers.

Pat Watt, a civic activist and past president of the Fairfax League of Women Voters, plans to announce her candidacy early next month, county Democrats said.

Herrity has not announced his plans, but he is expected to seek reelection.

"I don't think we have a candidate who is there just to keep Jack busy," said Fairfax Democratic Chairman Dottie P. Schick. "I think we have a very credible candidate."

The four-year terms of all nine county supervisors expire at the end of this year, and all nine are expected to seek reelection in November.

The filing deadline is April 15, and primaries, where necessary, will be held in June.

Eight of the supervisors run in districts, but the chairman is elected at-large in the 600,000-voter county.

Herrity, a former Democrat who is now a conservative Republican, was elected Springfield supervisor in 1971, won the chairmanship in 1975 and was reelected in 1979. He has maintained a high profile with blunt and often contentious remarks, and with tireless politicking at ground breakings, ribbon cuttings and weekend fairs across the 400-square-mile county.

"The only way she could beat him is with a big anti-Jack vote, and I don't think it's there," one political worker said.

"But at least she can keep him busy enough to keep him out of everyone else's district."

Watt, a native of Scotland, served as president of the Fairfax League of Women Voters from 1977 to 1979 and of the National Capital Area League in 1979 and 1980.

She has since served on county task forces on school closings, land use and water pollution and the county's retirement system. She is also president of her neighborhood Camelot Civic Association.

"There are a lot of problems with Jack, and there's no way to know when the constituency will become aware of them, or sufficiently aware of them," Schick said.

"Hopefully this will be the year."

The chairman's position, like all the supervisor jobs, is meant to be part-time.

Herrity operates an insurance agency, and Watt is a researcher with an association of municipal finance officers. Herrity declined to discuss the race yesterday and Watt could not be reached for comment.

So far there are announced opponents for only three incumbent supervisors--Mount Vernon Democrat Sandra L. Duckworth, Providence Democrat James M. Scott and Springfield Republican Marie B. Travesky--but party leaders have pledged that all nine seats will be contested.

In addition to those races, the county sheriff, prosecutor and clerk of court must run for election this year along with all state senators and delegates.