John F. Herrity, the son of an Arlington elevator maintenace man who worked his way through Georgetown Law School in two years by selling brushes, says he does not claim to be particularly bright.

He admits he has trouble reading speeches. His advisers have told him to wear a long-sleeve shirt while campaigning, but he will not do it, because he says that is not his style. He prefers to be called Jack, not John.

Herrity, the politician of humble roots, humble pie and sort-sleeve shirts, has managed to make himself one of the best-known elected officials in Northern Virginia and is widely considered an even bet to defeat incumbent 8th District Congressman Herbert E. Harris.

Harris, who won his seat in the so-called Watergate elections of 1972 and defeated a former Republican state legislator two years ago, "should be scared" of Herrity, says one of the congressman's long-time friends.

Confronting each other in debates, neither candidate look scared. They look angry.

Both men served together on the Fairfax County Board in the early 1970s, where they took turns trying to humiliate each other. "Quite simply, I think they hate each other," said Robert F. Horan Jr., the county's commonwealth's attorney.

Republican Herrity, who is chairman of the Fairfax County Board, is seen by many as a fearsome challenger to Democrat Harris because of Herrity's instinct for what are generally called "gut issues."

The gut issue that will get him to Congress is inflation, Herrity says. Throughout his 16-hour days in elementary school cafeterias and Rotary Club luncheons Herrity hammers away at inflation. When his eyes seem glazed with fatigue and his metaphors mix, Herrity's simple campaign claim still comes through: Inflation is bad and Herb Harris is making it worse.

Herb Harris, Herrity says again and again, is the biggest spending congressman in Virginia.

"The toughest job in Congress is to say no," Herrity says, "and very few people have the guts to say no." Herrity promises to say no.

According to his critics, saying no is nothing new for Herrity. That is how Herrity became a big name in the Virginia suburbs, they claim.

Fairfax Supervisor Alan H. Magazine, a democrat who disagrees frequently with Herrity, says Northern Virginians are generally content with the status quo and opposed to change. "Saying no is a good political stance in this area," Magazine says.

As Fairfax board chairman, Herrity led a community crusade against dirty magazines, a crusade that led to an ordinance that keeps the covers of Playbody and Penthouse from view in county stores.

For more than seven months this year, Herrity led a fight against a subsidized housing project planned for the Springfield area. Herrity claimed that the Rolling Roads Estates project, which was finally approved in the face of a threat by the federal government to deny Fairfax $3.8 million in federal aid, "would create a ghetto" in an area that already has its share of public housing.

Herrity says his position does not have anything to do with race or a penchant for nay-saying, as some county supervisors have claimed. "I think it is a sociological issue. I think it is a problem when you put people who are making $10,000 and $15,000 a year next to people making $60,000 or $70,000.

There is a jealousy that build up," Herrity says. One of the results, Herrity says, is crime.

Herrity, 46, a lawyer who gave up practicing law to sell insurance out of an office in his Springfield home, says he is a listener not a nay-sayer.

"I return every phone call I get. When people don't really understand the problems they complain about, I still talk to them and listen to their feelings," Herrity says.

Herrity, who was a democrat until 1968 when he decided that democrats were to liberal, fell into a political career in the mid-60s as a civic association president. "It just evolved. I certainly wasn't born into a political family. I was the only person from my family to go to college," says Herrity.

While continuing to work the 16-hour days he says have been routine for nearly 24 years. Herrity won election as a Fairfax supervisor from Springfield in 1971 and in 1975 he won a victory over an incumbent board chairman with a liberal record.

One reason why Herrity has become well-known in Northern Virginia is his willingness to speak out on a variety of issues.

An example, is the issue of whether the District of Columbia should retain its correctional facility at Lorton in Fairfax County: "Negotiating with the District is like negotiating with a marshmallow. Everybody is nice and sweet, but nothing ever happens."

To attract attention Herrity has twice brought toilets into the the county board room to discuss sewage issues and has gladly shown reporters a dog bite on his leg.

As the supposed leader of the county board, Herrity counts among his achievements pushing a four-lane extension of I-66 through Faifax and Arlington to the Potomac River, keeping the number of county employes down, attracting industry to the county and helping the county to tap the Potomac.

As Herrity admits, however, his leadership on the board has fallen apart in the past year. The chairman finds himself casting votes on the losing side of most major issues and has become the subject of ridicule.

Liberal members of the board have called Herrity a "demagogue," a later-day "Robespierre," a politician who "fans the fears of suburbia" and a man whose instincts are to assume the role of "the savior of the suburbs."

"I'm not a damn fool," Herrity says, referring to the collapse of his leadership on the board."I knew from the beginning [of his term] that leadership would crumble toward the elections. It has been a little difficult, but a lot of them [the supervisors] figure I'm going to Congress."

An issue that has not come up in the campaign this fall, but which was discussed by one of his two opponents before a June 13 Republican primary, is Herrity's health.

Just after he was elected board chairman, Herrity had two heart attacks. Since that time he has taken up running and job 3 1/2 miles every morning.

Herrity also lost weight but has not given up smoking despite a doctor's recommendation he do so. A cardiologist at Johns Hopkins University said this spring Herrity "should be allowed to continue running either with jogging shoes or for political office."

He has appeared fatigued during the final month of his nine-month run for Congress, but he says he is in better physical shape than his opponent Harris.

"Have you seen how tired Harris looks lately?" Herrity asks.