John F. Herrity wandered into the lobby of the Fairfax County Board chambers, wearing a canary yellow sports jacket buttoned taut across his thick torso, a thumb-sized buzz saw for a tie clip and sky blue knit pants that he hiked up from time to time. At 54, with a crooked nose and deep furrows framing his jaw, he had the air of a back-alley brawler out to dinner at the country club.

Chatting with a visitor, Herrity, the chairman of the County Board, was jostled by a man carrying a sheaf of papers.

"Excuse me, pal," said Herrity, jutting his chin in contempt. "Is it too much trouble to walk around me?"

The man sized up Herrity with the consideration a fellow might give to poking a skunk with a stick. Then he stalked off.

For 10 years, no one has seemed eager to pick a fight with "Jack" Herrity, a Democrat-turned-Republican who has scrapped his way to the top elected position in Fairfax County and won reelection by increasingly large margins.

In the past two weeks, however, Herrity has been confronted with what could be the biggest challenge in his political career -- allegations that he violated Virginia and county ethics laws.

Fairfax Commonwealth's Attorney Robert F. Horan Jr. said Friday he is expanding an investigation into whether Herrity violated county laws by voting on development proposals without disclosing his campaign donations from individuals tied to the proposals. Horan had been investigating Herrity's vote on another project involving a developer with whom Herrity owns an office condominium in Fairfax City.

If Herrity is prosecuted, it would taint his decade-long stewardship of the County Board with the first scandal in his political career and could cast doubt on his ability to continue leading Fairfax, the largest local government in Virginia.

The revelations already have reinforced Herrity's reputation as a friend of the region's booming development industry. That image could hurt him in next year's county elections, in which politicians and builders are predicting an antidevelopment backlash from voters angry at the daily traffic jams produced by the county's recent growth.

"I think there is definitely a feeling in Fairfax County that the citizens think that enough growth is enough," said County Board Vice Chairman Martha V. Pennino, a Democrat who is a Herrity defender.

The usually voluble Herrity has refused to comment publicly on the allegations against him, telling acquaintances that his lawyer has instructed him to keep quiet. Privately, Herrity has said that the burden should be on developers to disclose their contributions -- not on public officials to recall thousands of campaign donations.

Despite his troubles, few Northern Virginia politicians are discounting Herrity, the only Republican head of a local government in the Washington area.

"What you're going to have to remember is that Jack is a survivor," Pennino said. "The man has had three heart attacks and is still going strong. He'll make it, he'll survive."

Horan, interviewed shortly before Herrity's recent troubles, said, "Jack has shown a resilience. He lives and sleeps and breathes politics. He's a Richard Daley type in that sense."

In the past two years, emboldened by the first Republican-controlled board in Fairfax this century and driven by political instincts that leave his opponents nonplussed, Herrity has emerged as the dominant political figure in the county.

"In Fairfax," said prominent developer John T. (Til) Hazel, "he's the big gorilla."

In Virginia, Herrity has more constituents than any other local elected official, including the state's members of Congress. In the Washington area, he has become the personification of Fairfax County, a 399-square-mile jurisdiction with nearly 700,000 people -- more than the District of Columbia and about the same as San Francisco, the 13th largest city in the nation.

It was not always so. A native of Washington, Herrity entered the Coast Guard after high school and was later demoted two grades for fighting.

After his military service ended, he returned to Washington, entered college and graduated from Georgetown University Law School. He practiced law briefly and held a number of jobs before he was elected as supervisor from southwestern Fairfax's Springfield District in 1971, shortly after he left the local Democratic Party, which was torn by factions.

Then the only Republican amid eight Democrats on the County Board, Herrity was largely ignored by everyone but reporters, who found his off-the-cuff, sometimes crude remarks more quotable than those of his cautious colleagues. "I think we're moving toward 1984 10 years early," he quipped in 1974 when the board considered establishing a commission to investigate human rights abuses. When the board set up the panel two months later, Herrity labeled it "an invitation to snoopism, a vigilante operation."

In a dare to Democrats on the board, who rarely supported any of his initiatives, Herrity once proposed that the Pledge of Allegiance be recited before board meetings. He won -- on a split vote.

"I was hell on wheels," Herrity said of those days, smiling broadly.

In 1975, tired of not being taken seriously, he ran countywide for chairman against incumbent Jean Packard, a liberal environmentalist who had led a largely unsuccessful effort to slow the county's rapid growth. Herrity, supported by developers and business persons angry with Packard, defeated her with a little more than 51 percent of the vote.

Athough Herrity has long been a champion of development, there have been recent signs that he is trying to distance himself from the industry. This spring he voted for an increase in building inspection fees that was opposed by developers, and he has opposed dense development around the county's Metrorail stations.

Nonetheless, Herrity has been sympathetic to developers for most of his political career, and as an election year approaches he is taking more heat on the issue.

Appearing on a radio talk show last month to support an "outer beltway" to traverse the outskirts of the Washington suburbs, Herrity listened in silence as one caller castigated him.

"I don't think Mr. Herrity has ever seen a superhighway he didn't envy or a developer he wasn't in love with," the man said. "He seems even gleeful at the thought of yet another circle of congested development . . . . Now he wants more noise, more aggravation, more subsidization of developers who contribute very heavily to his reelection campaigns . . . . "

Herrity sidestepped the attack, suggesting that the man "hold a public hearing."

The caller was right on one point. Herrity's countywide campaigns have benefited substantially from the development industry. In 1983, about half of his $60,000 in campaign contributions came in chunks of $100 to $500, and much of that came from developers, development lawyers, real estate agents and others in the business. With their help, he outspent his opponent, former Fairfax Democratic chairman Pat Watt, by about $10,000.

"Jack probably has represented the developers' interests over the years more than anyone else, and he's been very effective doing it," said Supervisor Audrey Moore, a Democrat on the County Board and an opponent of rapid growth who is Herrity's nemesis. "He sincerely believes in development, and he hasn't wanted to see a lot of red tape -- what I would call planning -- in the way."

Herrity's ready response is that nothing can stop Fairfax's growth, that public officials can determine only what kind of growth will occur, and that his policy has been to attract more business and industry, which should reduce the tax burden on homeowners and apartment tenants.

"We were on the verge of becoming a wall-to-wall" subdivision, Herrity said in an interview. "The basic question is not whether Fairfax County is going to grow or not. It's how we're going to grow. The difference is that with economic rather than residential development, you have more money to pay for the roads that you need."

Many who have dealt with him for a decade say that Herrity's role on the board has evolved. "He's grown into an institutional leadership role," said Michael S. Horwatt, a prominent Democratic zoning lawyer who has supported Herrity. "He's gone from being a maverick who took potshots into a seasoned politician trying to get results as a legislator."

Herrity has achieved that status with a combination of fierce parochial pride in Fairfax, a habit of heaving brickbats at his opponents, and a knack for handling the news media as Leonard Bernstein might handle a symphony orchestra.

"When the cameras are going, Jack is going," said state Sen. Richard L. Saslaw (D-Fairfax), an adversary of Herrity.

The effect of Herrity's strident public persona has been to establish an identity, if still indistinct, for a county whose extremes of wealth and poverty, urban and rural areas, older and newer neighborhoods have in the past eluded even the appearance of unity.

Asked his opinion of his home town, Herrity replies, "I go to D.C. when I have to." New York? "Why would I spend my money to go to New York?" he asked. In Fairfax, he said, "I find pretty much everything I need . . . . I'm not exactly a famous traveler."

If Herrity's creed verges on chauvinism, as even some of his friends concede, it also has helped get him elected three times to the County Board chairmanship -- the last time, in 1983, with more than 54 percent of the vote.

Provided that he is not prosecuted, few politicians say they expect Herrity to have any serious trouble winning reelection to the $21,589-a-year post next year. He has increased his support by 10,000 votes in each of the past two elections, winning by margins of 13 percentage points and 9.5 percentage points.

Despite his heart attacks -- the last one, a year ago, was mild -- his next move up the political ladder has been a subject of speculation in both parties. He lost a congressional bid in 1978. Many say he will run again if Rep. Stan Parris (R-Va.) leaves Congress to run for governor in 1989.

Less plausibly -- given Herrity's parochialism, the current revelations and the suspicion with which Fairfax is viewed downstate -- others say he has his eye on the governor's mansion. Herrity will not say what he has in mind.

He does say, however, that he could make a lot more money by retiring from politics and devoting himself full time to his insurance business in Fairfax City, Jack Herrity & Associates.

Last year, he grossed more than $101,169 in commissions from pension and deferred compensation plans; after write-offs, his income from the firm was $31,198. Coupled with his county income, his total taxable income was $52,253.

Herrity would not reveal the names of his insurance clients. He said he does not knowingly sell insurance to companies or people who do business with the county or are involved in development. He said, however, that he has had to "bug out" of policies after selling them when he discovered a link with the county government.

He will not discuss his vote in February, granting waivers to county zoning regulations for 137 town houses proposed by Hersand Builders Inc. of Fairfax. Hersand's president and Herrity are partners in a Fairfax City office condominium. County and state laws require officials to disclose such relationships, and Herrity has said that he believes that his dealings with Hersand were exempt from disclosure requirements.

County Democrats seem at a loss to decide how to handle the current allegations against Herrity. None has attacked him directly, and their comments on the matter have been muted.

That is a contrast with the past, when Herrity's critics have not hesitated to assail him on the issue of public housing, which he generally has opposed despite a severe shortage of low-cost housing in the county; on the environment, about which they have charged he does not care, and on affirmative action, which in the past he has not embraced.

Although some say they detect some moderating of Herrity's stands -- he recently joined the NAACP, for instance -- most politicians in the two parties agree that, philosophically, he has changed little in 15 years of Northern Virginia politics.

"Jack's orientation is to go along with the power structure rather than to challenge it," said Herbert E. Harris, a former Democratic congressman and County Board member who defeated Herrity in the 1978 congressional race in Northern Virginia's 8th District.

Frustrated Democrats acknowledge that, like President Reagan, Herrity in the past has had a "Teflon coating" that leads voters not to blame him for their problems.

Moreover, his practice of appearing before any citizens group that invites him -- and a few that do not -- has won praise and votes.

On a Sunday in January, for example he attended a fund raiser in the District for the United Jewish Federation in the morning, a real estate open house in Reston after lunch, and a Fairfax Council of the Arts exhibition south of Alexandria after that.

In midafternoon he was at a building presentation in Springfield, followed by a trip downtown to present a proclamation for a Cerebral Palsy Telethon at TV Channel 5. At 6 p.m., he was instructed by his scheduling aide to "meet your family Herrity and his wife Justine have five grown children at the Mount Vernon Inn." It was probably a hasty encounter; he was due at a reception that same hour with D.C. Mayor Marion Barry at the Kennedy Center.

Last year Herrity drove about 15,000 miles to attend meetings and functions around the county.

"You can say that you're tired of seeing him," said Toni Carney, a former School Board member. "But tomorrow, if a neighbor paints his house chartreuse, you're going to call Jack Herrity. When voters need someone to be irate for them, he's the first person they'll think of."

Like New York Mayor Edward I. Koch, with whom he is sometimes compared, Herrity's politics is based largely on gut-level instincts and a knack for choosing issues that voters care about.

On Lorton, the troubled, District-run prison in southern Fairfax County, Herrity has repeatedly slammed the Barry administration for its management shortcomings. In the process, some officials say, he has inflamed local fears that an escape would endanger surrounding neighborhoods and raised false hopes that the prison will someday be closed.

"Love's not the only emotion that people are moved by," he said, defending his approach. "Sometimes it's fear."

"The suggestion that Fairfax County is going to do something about removing the prison is a pipe dream," said Horan, the county prosecutor. "I think he deludes people when he tells them he's going to do something."

Nonetheless, Herrity rarely misses a chance to criticize the prison and, by implication, the District government. "Lorton's a disaster," he said recently, explaining why he harps on it at every opportunity. "Sometimes you've got to hit a donkey over the head to get its attention."

Other donkeys in Herrity's book have been Metro, the regional transit service he regards as inefficient; Loudoun County, which he has repeatedly urged to help widen an important road that links the two counties, and Arlington County, which he regards as an urbanized mess.

Some politicians say Herrity's constant bashing of his neighbors may win him votes at the expense of regional harmony. "The notion that you don't have to work in some sort of spirit of cooperation is just crazy," said Harris, the Democrat who beat Herrity in the congressional race. "It makes for good stump speeches, but it makes for lousy government."

Similarly, Herrity's frequently blunt lobbying efforts for Fairfax in Richmond, particularly on local road funding options, made him persona non grata in the state capital, where cordiality is considered not just the measure of a gentleman, but also a valuable tool in getting things done.

After it became apparent to many people that Herrity was doing more harm than good with his demands for more local authority to borrow money for roads, the county adopted an unofficial policy discouraging board members from visiting Richmond when the legislature is in session.

"A very abrasive approach and personality doesn't fly well in Richmond at all," said state Sen. Clive L. DuVal, a Democrat who is dean of the county's delegation in the General Assembly. "His personality is an aggressive personality, and aggressiveness on the part of local officials is not appreciated down there."

What offends in Richmond has not harmed him in Fairfax. In supermarkets, housewives and men in work clothes greet him as "Jack." His office on the 11th floor of the Massey Building in Fairfax City handles a torrent of calls from residents every week. Viewing himself as an ordinary man of suburbia, Herrity takes many of the calls himself.

"I certainly don't view myself as very glamorous," said Herrity. "I'm about as common and ordinary as you can get and still be on two feet."