James G. Burton trudged up another set of stairs in the new condo complex, knocking on one door after another. Although he has spent four years representing this part of Loudoun County on the Board of Supervisors, he has gotten a lot of blank stares.

The neighborhood, South Riding, didn't exist during the last campaign. And about two-thirds of the 10,000 voters in his district weren't around when Burton was elected in 1995.

"A lot of people haven't been around long enough to keep track of what you're doing," said Burton, an independent. "People who don't have a track record seem as appealing as those who do."

It's not usually this hard for an incumbent politician to connect with voters. But Burton is a supervisor in the third-fastest-growing county in the nation.

The population boom in Washington's outer suburbs has obliterated the powerful advantages of incumbency in some races. Some officials who have served in office for years in fast-growing parts of Loudoun and Prince William County say they are no better off than challengers and are making their first contact with many voters.

This trend has played out on a larger scale in rapidly growing states in other parts of the country. In Oregon, for instance, political scientists said the influx of new voters left politicians scrambling to understand their politics and what issues they cared about. That has the effect of injecting a huge dose of uncertainty and concern for incumbents, they said.

"Incumbents have to be quite concerned when they see that kind of population growth," said James Gimpel, an associate professor of government at the University of Maryland who has studied the impact of population shifts and voting patterns. "They don't quite know what those kind of people are going to be like and what they're going to do on Election Day. . . . It's unbelievably unnerving."

In Prince William, state Sen. Charles J. Colgan (D) has about 95,000 voters in his district--nearly 23,000 more than when he ran four years ago. Colgan has served in public office since 1972, when he was a county supervisor, yet he's finding that many new residents have no clue who he is.

This is what Colgan hears when he goes door-to-door: "About half the time, in the new areas, they'll say to me, 'You're Senator who?' It's challenging. It's my job to get out and educate these folks."

His opponent, Robert S. FitzSimmonds III (R), said one reason he got in the race is that all the new voters are diluting Colgan's support. Colgan said a 1996 measure in Virginia allowing voters to register when they obtain driver's licenses has exacerbated the trend.

In Loudoun's Mercer District elections, development has clearly sparked the voter boom. The district includes a mix of new subdivisions near Fairfax County and centuries-old stone houses closer to West Virginia.

As of September, the district included 10,197 voters, up from 5,299 in 1995. Officials with the local Democratic and Republican parties said that about 3,400 of the current registered voters were around in the last election.

A large number of them are moving from Fairfax, have young children and are working in technology jobs. But the district also includes many longtime residents and a heavy dose of the equestrian crowd in the Middleburg area.

Four years ago, the South Riding community was farmland. Now there are nearly 2,000 houses and apartments. Eventually there will be almost 6,000.

The influx of new residents has left Burton, a 62-year-old retiree endorsed by the Democratic Party, scrambling to get his message out to people who aren't familiar with his record, notably his efforts to slow development.

But the changing population also has created an opening for his Republican challenger, Mark Tate, 34, a restaurant owner who has worn holes through his scuffed penny loafers walking to so many houses.

Both candidates are knocking on lots of doors. They are targeting mailings to new voters. They have both launched extensive Web sites and are fielding a flood of e-mail inquiries about their positions. And they are both preaching a message of slow growth--ironic given that many of the new residents have moved into new houses.

On a recent Saturday, Burton went door-to-door in South Riding while his wife, Lina, waited in their Subaru Outback. She handed him a map and list of registered voters and he was off, climbed the stairs of green-shuttered condo developments.

He's visiting every registered voter in South Riding, not just the ones who voted in previous elections, because he wants to reach new residents.

"I'm on the Loudoun Board of Supervisors running for reelection as an independent," the candidate said, extending his hand as residents opened their doors. "I ask you to check out my credentials."

Burton, sporting white slacks, a cream sweater, walking shoes and a mane of gray hair, encountered Fran Husdale, 67, walking toward her garage. He handed her a bookmark with his picture, a synopsis of his record and efforts to slow development.

"The original slow growth advocate," the bookmark said. "Voted FOR every growth control measure and AGAINST 93% of new houses."

Burton also chatted with her about his experience in the Air Force.

Husdale, who said she is registered to vote, hadn't heard of Burton. But she liked what he had to say. "I'd like to vote," said Husdale, who moved in three years ago. "I'm just not as familiar as I should be."

That should have been Burton's last stop in South Riding, where he has been campaigning since July. But since he started, 100 new houses have gone up. He doubled back to pound on more doors.

Tate and the Republicans also are going after new voters. Tate recently went door-to-door in Courts of Vanderbilt, a Leesburg town house community. Wearing a rumpled white shirt, black-and-yellow tie and khaki slacks, Tate knocked on dozens of doors in the late afternoon. He went to every door, he said, because the latest lists of registered voters quickly grow outdated and he did not want to miss anyone.

Tate talked up an endorsement by the Loudoun Education Association, which seemed to play well in this neighborhood filled with tricycles, swing sets and children romping on the street. He passed out brochures touting a 10-point growth-control plan and his service on the Middleburg Town Council.

His message appealed to newcomer Sunshine Ricketts, 29, whose daughter Elizabeth looked on as Tate made his pitch. Ricketts said she had no idea who was on the ballot. "Other than the signs, I haven't seen much," Ricketts said.

In addition to encountering few people who know Burton, Tate also enjoys another unusual benefit for a challenger: He has raised more money then Burton.

"We both have the same opportunity to get our message out," Tate said. "The four years my opponent has been sitting on the board don't register."

Still, there are plenty of new people who have yet to hear about either candidate.

Patricia Broderick, 36, and her family moved to the Leesburg area last month. They're still unpacking. And Broderick, a homemaker, has been too busy for politics.

"It's something I haven't been able to get to," she said. "I'll eventually figure it out. Let me tell you, this has happened many times before. I've stayed up the night before reading through the issues."