It was early on a drizzly, raw weekend morning in Fairfax County. Few people were on the sidewalks, few in the stores. For once, the glistening streets were free of rush-hour traffic.

Suburban politicking thrives on weekend mornings like this. In a low-slung yellow brick office building near Baileys Crossroads, Thomas M. Davis III, Republican supervisor for Fairfax County's Mason District, listened to a handful of his constituents. They wanted $10,000 for new uniforms for a high school band.

Davis, a 37-year-old lawyer, leaned forward in his shirtsleeves across a conference table, nodding vigorously in assent, knitting his brow in concern, penciling notes on a legal pad.

"I think we can get some money from the local developers," he assured his visitors in the swift, self-assured manner that is his trademark. "I'll call these guys. I can bother them."

In Fairfax, where taxes and traffic tend to outweigh partisan issues in most local elections, Davis has emerged as one of Northern Virginia's most popular young politicians. By assiduously tending to the needs of the 80,000 residents in his inside-the-Beltway district, Davis has been able to win -- and win big -- despite the growing strength of the Virginia Democratic Party.

Today his political skills will be tested as he marshals a group of moderate Republicans trying to oust Fairfax's conservative Republican chairman, Benton K. Partin, at a county GOP convention expected to draw 3,200 people. If the plan works, politicians say it could leave Davis in a strong position to succeed Fairfax County Board Chairman John F. Herrity, if he steps down or seeks higher office.

Davis acknowledges that he would like to succeed Herrity in the county's top elected office, a job with as much local visibility as a congressman. That aspiration has led to a rivalry between Herrity and Davis that occasionally erupts into one-upmanship and verbal scrapes during supervisors' meetings.

"Everybody likes Tom," said Jeannie Meyer, who with Davis' help managed to raise most of the funds needed for the new band uniforms. "I don't know anybody who's been disappointed if they've asked him for help."

Critics say Davis' efforts to keep all sides happy have left some with the impression that all he cares about is his own advancement. "He's an ambitious young man," said Anne A. Wilkins, a Democrat who represented Mason District from 1952 to 1964. "And that's his principal interest: political ambition."

"Voters like someone who stands for something, come hell or high water," said Alan H. Magazine, the Democrat who preceded Davis on the county board. "Tom Davis . . . has been accused of telling people one thing and turning around and telling other people the opposite. It's a theme."

Davis counters that he is a pragmatist, by nature uneasy with political ideology. As an undergraduate at Amherst in the restive 1960s, he supported the civil rights movement but was wary "of the self-righteousness of the left . . . . The passion, the self-righteousness, the morality was a turnoff for me."

He also acknowledged what most politicians who know him have noted: that he is a political junkie, fascinated with the mechanics of elections, able to tick off precinct voting patterns and results from virtually any locality in Virginia.

"He's like a human computer," said Martha V. Pennino, a Democratic supervisor from Fairfax's Centreville District. "He knew what my returns were 18 years ago, which I found rather interesting. It's a tremendous help for him."

Many Republicans say Davis is the major power behind former county sheriff James D. Swinson, who is attempting to defeat Partin at today's convention. If Swinson wins, these critics say, Davis and a handful of others can effectively control the Fairfax GOP from behind the scenes.

Davis denies the allegation, saying he wants to oust Partin, whom he considers a right-wing ideologue, for the good of the party.

By most accounts, including his own, Davis would rather be running for Congress or chairman of the county board. For the time being, however, both options seem precluded by Republican incumbents.

"I am really boxed in, in terms of upward mobility," he said in an inteview. "I certainly support the people ahead of me, like Rep. Frank Wolf of the 10th District and Jack" Herrity.

"I don't think I'll stay as a 20-year supervisor," Davis said. "If staying in office is just a waiting game for you, that's when it becomes too long, if you don't have an agenda you're working on."

Davis' agenda, most Fairfax politicians agree, is shaped largely by pragmatism and an overriding strategy to build on his base in Mason District, a chunk of thickly populated territory wedged against Alexandria and Arlington in eastern Fairfax County.

Although Davis has regarded himself as a Republican and a conservative since his high school days as a clerk in the U.S. Senate (he has more than 50 wooden, ivory and rubber elephants on display in his office), as supervisor he has championed some causes traditionally associated with the Democratic Party, such as affordable housing and shelters for the homeless. Moreover, he has appointed a number of Democrats to county commissions and advisory boards, a policy that irks some Fairfax Republicans.

That strategy, say all but a few GOP hard-liners, is shrewd in a swing district such as Mason that has frequently voted for Democrats. The issues Davis has chosen to emphasize have particular appeal to voters in Mason, which many liken to the county's urbanized "inner city."

In Mason, Davis faces few of the questions generated by the tumultuous growth that has rocked Reston, Centreville and other areas in western Fairfax. Rather, his primary challenge has been to ease the redevelopment of aging parts of his district, such as Seven Corners and Annandale.

In Seven Corners, he has been widely praised for helping to revitalize the area, attracting money for road improvements, persuading condominium developers to finance the relocation of tenants and soothing adjacent homeowners who feared the effects of infringing commercial development.

"Tom's in touch with people all the time," said Bill Evans, the Mason District Republican chairman who has clashed often with Davis on party matters. "Tom goes one step further and will call people for no particular reason, not because there's a problem but because he wants to head off problems."

Politics came early to Davis, who grew up just outside Mason District in Arlington. His grandfather, a Nebraska Republican, was an under secretary in the Interior Department under Eisenhower. Using that connection, Davis was appointed by the two Nebraska senators to the Capitol Page School at age 14. He became the head Republican floor boy in the Senate and president of his graduating class of 20 students.

At home, he was brought up by his mother. His father, an alcoholic who had periodic run-ins with the law, served time in state prisons. To this day, Davis said, he has never tasted alcohol.

Entering Amherst in 1967, he was out of step politically with many of his classmates. In a 1968 campus mock election, he said, presidential candidate Richard Nixon placed third, behind Hubert Humphrey and Eldridge Cleaver. "The middle of the road was down the left-hand gutter," he said.

He spent the fall of his senior year as an intern in the Nixon White House, then a year after graduation in the Army.

As a student at University of Virginia Law School in 1973, he married Peggy Rantz, a medical student whom he had known since his days as a Senate page. He also worked as an aide in the House of Delegates to a Northern Virginia legislator, Wyatt B. Durrette, commuting to Richmond during the legislative session. A decade later, Davis was chairman of Durrette's Northern Virginia advisory committee during his unsuccessful race for governor.

Davis returned to Northern Virginia after law school, was the county's GOP chairman briefly and ran for the county board in 1979 when the Democratic incumbent, Magazine, decided not to seek a third term. Davis' Democratic opponent, Betsy Hinkle, was dying of cancer during the race, and although her illness was not widely known, she was unable to campaign energetically. He won easily. In his race for a second term in 1983, he won again, this time with 77 percent of the vote.

Since his election to the part-time supervisor's position at an annual salary of $21,589, he has become general counsel for Advanced Technology Inc., a high-technology defense contractor in Reston.

The job, coupled with his wife's medical practice, gives the couple a joint annual income of about $175,000. They live in a large brick house near Lake Barcroft with their two children, Carlton, 4 years, and Pamela, 8 months. Davis' study is lined with volumes of "America Votes," a reference book recording decades of election results.

Durrette's loss in last fall's gubernatorial election capped a long-simmering dispute between moderate Republicans such as Davis and the Fairfax County GOP leadership, controlled by Partin.

Davis said that Durrette, his former boss, fared poorly in Fairfax partly because Partin had estranged grass-roots organizers in his zeal to impose right-wing views on the county Republican Party.

Early this year, Davis and Supervisor Nancy K. Falck of Dranesville rallied the opposition to Partin and drafted Swinson, the former sheriff, to run.

Since then, the war of words has escalated. "I think Tom's the one who's aroused the polarization," said Partin.

Davis rejects charges that he has polarized the party. "I certainly don't consider myself an ideologue . . . . I have a long-term commitment to the community. I've established roots here. That's what I stand for."