This was a pop quiz, and the woman standing at the doorstep of her Prince William County town house was given no time to prepare. The question: Who is your county supervisor?

"Yes, I know," said B. Harkins, a former congressional staff member and a two-year resident of the Lake Ridge community. "I know, but I can't think of his name."

Whoops. Her name is Kathleen K. Seefeldt, and, with 12 years' experience, she is the senior member of the Prince William Board of County Supervisors.

By all standard political logic, Seefeldt should be a familiar name in one of the most affluent and well-educated sections of the county. Yet in an informal survey last week of 20 households in the newest areas of Seefeldt's Occoquan District, where many of the houses were not yet built in the last election four years ago, only one person could name the Democratic incumbent. However, that is one more than could name Gregory L. Cebula, Seefeldt's Republican opponent in the Nov. 3 election.

In Northern Virginia, where thousands of new residents join the voting rolls with each election campaign, traditional political truisms often can be cast to the winds. In Fairfax County -- and to an even greater extent in outer suburbs such as Loudoun and Prince William -- new residents have reshuffled the political deck for this election.

Longtime incumbents, who in most places must suffer a scandal or commit a particularly egregious blunder to endanger their reelection, find themselves running as if they were political newcomers. Even in high-profile elections such as the race for the Fairfax County board chairmanship, generally well-known candidates such as Republican incumbent John F. Herrity and Democratic challenger Audrey Moore are scrambling to win over the more than 94,000 new voters since the last county election four years ago.

"That's what's scary about an election -- it's the people I don't know and who have had no personal contact with me or my opponent," said Prince William Commonwealth's Attorney Paul B. Ebert, who is facing perhaps his toughest race in two decades as the county's top prosecutor.

New residents have been reshaping the institutions and customs of Northern Virginia politics for more than 20 years. A flood of suburbanites beginning in the 1960s was largely responsible for the emergence of a viable Republican Party in Fairfax County, which, like the rest of Virginia, was once a Democratic monolith, according to many political analysts.

At the same time, the focal points of political activity have changed. In the past, an aspiring officeholder would spend time chatting up party regulars at a popular club or restaurant. These days, a shrewd politician in the suburbs can be found shaking hands at youth soccer games and meetings of parent-teacher associations and homeowners groups.

The influence of new voters this year may be more powerful than ever, according to political observers across Northern Virginia. Lowered interest rates and a healthy regional economy have produced a housing boom, in turn bringing about a 34 percent increase in the number of registered voters in Fairfax, 32 percent in Prince William, and 26 percent in Loudoun.

These voters bring new tastes, new priorities and new grievances to the political process. And, to hear many incumbent officials tell it, the recent arrivals bring their share of political ignorance.

Anonymity is enough to bruise the self-esteem of any politician. Yet many of the new residents not only do not know names and faces but also are unfamiliar with basic facts about local government, some officeholders maintain.

Prince William County Supervisor Edwin C. King, a Dumfries Democrat running for reelection, said he often must give impromptu civics lessons on the campaign trail.

Someone newly relocated from New York may not know of Virginia's adherence to the Dillon Rule, which sharply limits the power of local governments to control growth. A former Minnesotan outraged about the long commute from Prince William to the Pentagon may not sit still for an explanation that the state government, not local supervisors, is in charge of Northern Virginia's overburdened road network.

"It's not that the new citizen doesn't know who Ed King is -- I can fix that through campaigning," said the incumbent, who is running against Norma G. Pandazides, an independent who has been endorsed by Republicans. More difficult, King said, is to convey, without seeming uncaring or ineffective, that there are many problems a local official cannot solve.

Fairfax County Supervisor Thomas M. Davis III (R-Mason) agrees that new voters have heightened expectations of local government and public schools. "They want results, not excuses," said Davis, who is facing only token opposition from independent Daniel Belsole in next month's election.

All this can spell trouble for incumbents, whose natural advantage can be sharply diminished in a neighborhood that a year ago was a dairy pasture.

Supervisor Martha V. Pennino has represented the Centreville District on the Fairfax County board since 1968, prompting many residents to dub her "Mother Fairfax." Republican strategists, however, maintain that the young engineers, consultants and others who in recent years have streamed to the high-technology corridor near Reston are unlikely to know the Democratic incumbent, much less view her as a maternal figure. They claim that these new, conservative-leaning voters give their candidate, Linda Douglas, a decent chance of upsetting Pennino.

The prudent course, many politicians say, is to campaign relentlessly, no matter how many years an incumbent has served. Because of new voters, "a politician has to be visible in the community at all times," said Prince William Supervisor John D. Jenkins (D-Neabsco), who added that he always campaigns as though the race were neck and neck. Even by the standards of Northern Virginia, Jenkins' caution is surprising: He is unopposed in next month's election. "I don't want to be surprised by a write-in campaign," said Jenkins, whose enormous frame and arm-pumping style are a fixture at youth athletic contests, ribbon-cuttings and seemingly any other event in his Dale City district where more than two people are likely to gather.

But even outer suburbs such as Prince William have become too large for a candidate to rely strictly on person-to-person campaigning. Northern Virginia politics has entered the media age, which has dramatically increased the role of money.

The Herrity-Moore contest already has broken the spending record for a local race in Virginia and may wind up totaling more than $750,000, with much of the money going for mass media advertising. Ebert hopes to raise about $70,000, a Prince William record, using the money for a cable television blitz in his race against GOP challenger Peter W. Steketee.

The life style of Northern Virginia's outer suburbs adds another twist to next month's elections. Many new residents of Loudoun and Prince William are two-income couples who leave their homes by 6 each weekday morning, and when they return at night they are too tired or preoccupied with bill-paying and children to pay much attention to local politics.

At a recent meet-the-candidates night in Prince William, 11 candidates and 22 residents turned out -- a ratio that would pierce the ego of any politician, particularly because many in the audience were related to one of the candidates. Rather than attend debates and forums, many residents learn about the issues and candidates from a variety of informal sources, some of them unorthodox. Harkins, the former congressional worker, said she gets the lowdown on Prince William politics from the guy who works on her yard. One of her neighbors, Patti Scronce, said her children's baby sitter told her that a school bond proposal on next month's ballot deserves a "pro" vote.

Michael and Maurina Rachuba moved to Prince William last year because they were able to buy a far larger town house there than they could afford closer to their federal jobs in Crystal City and the District. Like many of their neighbors, the Rachubas acknowledged a trifle sheepishly that they have been too busy to follow local politics, but they said they plan to immerse themselves in the issues in the last day or two before the election.

Keith Breedlove, who moved with his three young daughters to a new block in the Twin Oaks Farm subdivision a few months ago, exemplifies the changing priorities new voters impose. Breedlove said he was amazed that Prince William voters, traditionally conservative on fiscal matters, have repeatedly rejected bond proposals to finance new roads and other public facilities, adding that he will support next month's school bond referendum.

New voters also may seek different political manners and values in their elected officials, and some candidates deliberately style themselves to capture their allegiance. Denis Catalano, an independent running a long-shot campaign for supervisor in Prince William's Coles District, has denounced "the good old boys" who supposedly control county government.

In fact, when Seefeldt was first elected in 1975, she was widely viewed as the new voters' "good government" alternative to what many believed was a clubbish and incompetent clique running the county. Now, in her first contested race since then, she said she hopes to win reelection by explaining to the county's latest voters the legal constraints on the authority of local officials, then extolling her accomplishments.