Northern Virginia registrars estimate that hundreds of potential voters were not able to cast ballots Tuesday because of lingering problems with the federal "motor-voter" law passed in 1993 to boost participation in elections.

People believing they had registered to vote at the Department of Motor Vehicles or another state office showed up at polling places Tuesday only to be turned away because their names did not appear on official voter rolls, which close to new additions 30 days before elections, the registrars said.

"This is a system with inherent flaws in it," said Fairfax County Registrar Robert W. Beers. Registration applications "are going into the hands of a bunch of DMV counter clerks. I don't know what their commitment to voter registration is. . . . Too many people, too many places, too much handling."

Beers estimated that the law might have cost hundreds of people the chance to vote in Fairfax County. Other Northern Virginia registrars reported a smaller number of complaints.

State officials acknowledge problems with the National Voter Registration Act of 1993--usually called the motor-voter law--which former governor George Allen fought in federal court before agreeing to implement in 1996.

Officials say many problems have been resolved since then. Most of the remaining complaints, they add, are concentrated in Northern Virginia and stem from the failure of would-be voters to fill out forms completely or to sign them.

"I think it's fair to say that statewide it's not really a problem. But it does seem to be a problem in Northern Virginia," said Cameron P. Quinn, secretary of the state Board of Elections.

The motor-voter law allows registration at the DMV, welfare offices and other state agencies. But applications that are not signed go to the shredder.

State officials say that since 1996, the law has helped swell Virginia rolls by 1.1 million voters, to 3.8 million. About three in every four new voters register through motor-voter, though Quinn said the number of people actually voting has not increased, when adjusted for population growth.

"It's really doing what I've been trying to do since I got appointed, which is get more people registered," said Alexandria Registrar Beverly Beidler, who added that her office got fewer than 10 complaints from would-be voters kept away by administrative snags.

DMV Commissioner Richard D. Holcomb said his agency has worked hard to simplify motor-voter. Last month, Prince William County became the second of two pilot sites for a program in which local election officials have a window in DMV offices. Manassas and Alexandria are on the list to get a similar window to help answer questions and smooth the registration process.

"We are obviously very committed to make this process work," Holcomb said. "The registrars are right. It is a cumbersome process. But the bottom line is it is working."

Republicans, who have long been critical of the program, said today that if hundreds of voters couldn't cast a ballot in Fairfax County on Tuesday, it could have affected the outcome of the close race in which Democrat Leslie L. Byrne defeated Sen. Jane H. Woods (R-Fairfax). That district includes part of the county.

"If several hundred people were prevented from voting, that would have had a huge impact," said GOP spokesman Tim Murtaugh. "There's no way of knowing who those people would have voted for." But Democratic Party Executive Director Craig K. Bieber said, "This is purely a case of Republican sour grapes because they lost the election."