In Fairfax County, where attacking the controversial sheriff's department is almost as popular as bemoaning the price of gasoline, the two Democratic candidates for sheriff in next Tuesday's primary subscribe to different schools in the art of tarring the Republican incumbent.

The target of the Democrats is Sheriff James D. Swinson, the most successful Republican vote-getter in Fairfax history. Swinson, who is not seeking reelection, has been widely accused of callousness and incompetence in running the county jail.

Terry C. Armstrong, one Democratic candidate for sheriff, is pulling no punches in his assault on Swinson. He says the sheriff is responsible for the deaths last year of three prisoners who died after confinement in the jail. He accuses Swinson of "nepotism" and "cronyism." He says the first thing he will do, if elected, is fire the "bad apples" who work at the jail.

Kenneth R. Wilson, the other Democratic candidates and the recently retired deputy chief of the Fairfax County Police Department, is more circumspect. He speaks of certain "irregularities" at the jail that occurred because the "current administration has not made the hard decisions."

Wilson, 46, does not promise to fire anyone until he can get a close look at the jail operation. He says that his opponent, Armstrong, 39, is one of the those "people who will say anything to get a vote.

"My guns are loaded [as far as attacking Swinson], but I don't want to shoot it all on this primary," Wilson said. He said he expects to easily defeat Armstrong and wants to save some of his allegations about the jail for the general election in November.

In that election, the winner of the Democratic primary will face one of three Republicans competing in that party's primary next Tuesday. They are M. Wayne Huggins, the current chief deputy in the sheriff's department; Myron L. (Bud) Greenquist, a former chief deputy in the department who was fired by Swinson, and Howard L. Miller, director of a security company.

Both Democratic candidates are hoping that Huggins, considered by many Republicans to be a slight favorite because of his support from Swinson, wins the Republican primary because Huggins has been connected for the past year in the problems of the sheriff's department.

"Mr. Huggins is just immersed in all this scandal. I would love to face Mr. Huggins," said Armstrong, 39, who quit his job as a corrections officer for the District of Columbia four years ago.

Armstrong said he quit the job "soley to go to school and get all the degrees necessary to do the sheriff's job." he has earned degrees in administration of justice and corrections from Northern Virginia Community College, American University and George Washington Universtiy. The campaign photograph that Armstrong has circulated recently pictures him sitting in front of his diplomas from these colleges.

Democratic Party regulars in Fairfax have thrown their support behind Wilson, who has run what one politician says is a "strong and steady" campaign compared to Armstrong's more "unpredictable" effort.

With election board officials in Fairfax predicting a turnout of about 10 percent of the 243,813 registered voters for the Democratic prmary, the support of party regulars could swing the election.

Yet, many Democrats in the county say that Armstrong has run a more "visible" campaign and that it is difficult to determine how much recent publicity about the sheriff's department has affected voters.

For his part, Armstrong is trying to arouse as many voters as possible. To do so, he has taken to attacking his Democratic opponent along with the Republican sheriff.

Armstrong accuses Wilson of being "buddy-buddy" with the law enforcement establishment in the county and says that if Wilson is elected, county taxpayers will have to pay him a salary second in the state only to Gov. John N. Dalton.

If Wilson is elected, his annual income, combined with his retirement pay of $18,000 for 23 years with the county police, will begin at $52,000. The sheriff's salary starts at $34,000 a year and increases by steps to $48,000 after 10 years. Gov. Dalton makes $60,000 a year.

"I don't think the county needs to be paying two salaries to a former policeman," Armstrong said. "This is double-dipping."

Wilson said that his retirement pay, which he says he earned by service to the county and contributing to the pension fund, "isn't an issue."

What is an issue, according to Wilson, is his Democratic opponent's campaign brochures which claim that State Sen. Clive Duval (D-Fairfax) has endorsed Armstrong.

A spokeman for Duval said yesterday that the senator is planning to vote for Armstrong, but that Duval did not give Armstrong permission for an endorsement that was in a brochure mailed last week to 30,000 homes in the county.

Armstrong said yesterday he made a mistake based on a misunderstanding with Duval's campaign manager.

Election officials in Fairfax estimate that a maximum of 48,000 voters will make it to the polls on Tuesday to vote in either the Democratic or Republican primaries.

Mel Rappleyea, secretary of the election board, said yesterday that absentee ballots are running ahead of the election four years ago and that a high voter turnout is possible. CAPTION: Picture 1, KENNETH R. WILSON . . . retired Fairfax police deputy; Picture 2, TERRY C. ARMSTRONG . . . pulling no punches in campaign