The trickiest part about running in the Republican primary for sheriff of Fairfax County is what is say about the man who is vacating the office.

Retiring Sheriff James R. Swinson, 66, is under severe criticism for alleged administrative incompetence. he also is the most successful Republican vote-getter in county history.

The two major Republican candidates for sheriff - chief deputy M. Wayne Huggins and former number two man Myron L. (Bud) Greenquist - have resolved the problem by speaking neither good nor ill of Swinson. Mostly they don't mention him.

Still, in suburban primary elections, which frequently offer the excitement of watching grass grow, the race between Huggins and Greenquist to win the June 12 GOP election is seen by many observers as close and bitter.

"It's the only race we've got out here where there is a little bit of dirt being thrown around," said one Republican campaign official.

Their Democratic opponent in next fall's general election will be decided in the June 12 Democratic primry contest between former Fairfax deputy police chief Kenneth R. Wilson and Terry Armstrong, a former senior corrections officer for the District of Columbia.

Meanwhile, the two major Republican candidates for the job - for which Swinson is paid $42,000 annually one of the highest sheriff's salaries in Virginia - carefully avoid either riding or stomping on Swinson's political coattails.

"I can do my speaking for myself," says Huggins, who was hired by Swinson last year as the number two man in the department and who does not mention Swinson's name in his campaign speeches or in his campaign literature.

"I don't speak ill of Swinson, our differences are purely professional, not personal," says Greenquist - who was fired by Swinson last year as the number two man in the department after Swinson said the two could not work as a team.

For his part, Swinson, who has been sheriff for 16 years, is not saying anything. Swinson stopped talking largely in respond to a flood of criticism over the last year, during which three black prisoners died - one of them suffering acute alcohol withdrawal - after confinement in the modern, $4.6 million jail Swinson runs.

Two investigations of jail operations by Commonwealth's Attorney Robert F. Horan Jr., conducted at the request of county supervisors, also brought Swinson unwanted public attention.

But the silent sheriff, according to knowledge Republicans in Fairfax, still has considerable political influence, especially in primary elections where fewer than one out of every five registered voters in the county casts a ballot.

Many Republicans in Fairfax say Huggins and Greenquist cannot afford to attack Swinson for fear of angering the party faithful, who have disproportionate influence in a primary election because of low voter turn-out. And they cannot afford to be too closely associated with a sheriff who has been widely criticized for alleged incompetence and callousness in running his jail.

The only criticism of Swinson to emerge in the GOP race has come from a third candicate - Howard L. Miller, director of security operations for Air France Concorde at Dulles International Airport.

Miller, who has run a low-key campaign and is not considered to be a contender in the June 12 primary, has run newspaper advertisements attacking Swinson's integrity and operation of the jail.

Miller, according to one source in Greenquist's campaign, is saying things that Greenquist would say if it were not for Swinson's influence on primary voters.

In a dual appearance Wednesday night at the Falls Church Community Center, Greenquist instead took aim at Huggins.

Greenquist, 48, who spent 26 years with U.S. Army intelligence and three years with the Fairfax sheriff's department, accused Huggins of pressuring sheriff's deputies to work for the chief deputy's election campaign.

Huggins said that he would be "lying to myself" if he said that he didn't welcome the support of the 150 deputies, who traditionally have helped Swinson to get reelected despite their status as civil servants. But Huggins denied Greenquist's accusation. "I will not tolerate pressure being put on any of my deputies," Huggins said.

Huggins, 30, a former Virginia state trooper tries to concentrate in his speaking engagements around Fairfax on the future of the sheriffs department.

If elected. Huggins says, "the Board of Supervisors is going to get tired of hearing me" becayuse he is going to pressure the county to build a facility to handle chronic alcoholics. The chief deputy says a jail is not the proper place to handle such people.

While Greenquist doesn't disagree that alcoholics present a problem in the Fairfax jail, he argues that the sheriff doesn't have any power to do anything about it.

"The sherrif's job is administrative. He runs a department with a $3 million budget. The sheriff doesn't have anything to do with the drug scene or the alcoholic scene. For the past 39 years the police have handled law enforcement," Greenquist says.

And to demostrate that better control is needed over the jail, Greenquist brings up the adverse publicity from the past year.

"[Those things] happened and that is what is going to be on our back in the general election in November. I've been separated from those disturbing events that have happened and I can retain this office in the Republican Party," Greenquist claims.

Both Greenquist and Huggins are forceful public speakers, and Republicans in Fairfax say that many voters who have attended forums to hear the sheriff's candidates remain undecided. CAPTION: Picture 1, M. WAYNE HUGGINS . . . "speaking for myself"; Picture 2, MYRON L. (BUD) GREENQUIST . . . "I don't speak ill" of Swinson