Fairfax County Supervisor Audrey Moore increased her lead over County Board Chairman John F. Herrity as the campaign for Fairfax's top elected position entered its final days, according to a new Washington Post survey.

In a poll designed primarily to measure change of voters' sentiment since mid-September -- not to predict the outcome of Tuesday's election -- Moore appeared to have been more successful than Herrity in both retaining her supporters and attracting previously undecided voters.

Moore, 58, an Annandale District Democrat, led Herrity, 55, a Republican, among every identifiable group in Fairfax except Republicans.

The poll, completed Thursday, showed Moore with a lead of 20 percentage points over Herrity among those who said they are likely to vote, with 54 percent favoring her and 34 percent supporting Herrity. The remaining 12 percent of the respondents were undecided or favored another candidate.

Among all registered voters surveyed, Moore's lead was somewhat smaller, 52 percent to 35 percent, with 13 percent undecided or supporting another candidate.

A spokeswoman for Moore said the poll reflected Moore's momentum. A Herrity aide questioned the methodology of the survey and pointed out that other recent surveys, including one two weeks ago by The Post, have shown a tight race between the candidates.

"Polls have been wrong before and most of the time they are," said Herrity spokeswoman M. Constance Bedell. "The only poll we're concerned about is the one" on Election Day.

The Post poll did not ask about two independents in the race for county board chairman, Robert T. (Terry) Robarge, a Centreville mortgage banker, and James S. Morris Jr., an Oakton real estate broker.

Tuesday's election will cap a six-month campaign costing nearly $1 million, making it by far the most expensive race for local office ever in Washington's suburbs. The election is seen by most analysts as a referendum on the rapid development -- backed by Herrity and frequently opposed by Moore -- that has transformed Fairfax from bedroom community to economic dynamo in the last 12 years.

The poll "reflects the feeling we've been getting from the voters," said Moore spokeswoman Janice Spector. "We have the momentum and, most importantly, we feel we have the issue: Mrs. Moore has the clear understanding that transportation and development are linked."

Alex Castellanos, Herrity's media consultant, questioned the methodology of the survey, which he said produced "an incredibly wild swing {that} doesn't match all the other polling numbers we've seen, even from The Post."

The most recent poll, known as a panel-back in the parlance of pollsters, was designed to measure change in voter sentiment since a survey was conducted in September. A total of 899 of the 1,146 registered voters The Post had interviewed in September were contacted again to determine their current preference. The margin of sampling error on a random survey of 899 respondents is plus or minus 3 percentage points.

Castellanos, who has advised Sen. Jesse Helms (R-N.C.) and Sen. Robert J. Dole (R-Kan.), a presidential candidate, suggested that The Post's survey would "deal a serious blow" to Herrity's chances for a fourth term. He added: "The indication the campaign's been getting is that things have tightened up and Jack Herrity's on the move. We're going to have a close race." He said the latest poll would compound any inaccuracy in the September poll.

Moore's apparent advantage over Herrity represents a significant and somewhat surprising increase in her support since a Post poll in mid-September, which showed her with a 14-point lead among likely voters.

The new poll suggests an even more abrupt shift since a Post survey two weeks ago found Moore leading Herrity by 7 percentage points among likely voters. A survey conducted by the suburban Journal Newspapers at about the same time reported that she held a 5-point lead over Herrity.

Such volatility is often characteristic of local political races, in which candidates' advertising, breaking news reports or last-minute endorsements can play an important role in determining voter sentiment. In addition, voter turnout, which is expected by the candidates and others to be 40 to 45 percent, could dramatically affect the only numbers that matter: the balloting Tuesday.

In November 1985, a Washington Post panel-back poll published two days before Virginia's statewide elections showed Democrat L. Douglas Wilder leading his Republican opponent for lieutenant governor by 24 percentage points. Wilder did win on Election Day -- but by a margin of less than 4 points.

Interviewing for the latest Post survey began last Tuesday, exactly a week before Election Day, and ended Thursday as both candidates' expensive eleventh-hour media blitzes hit full stride.

Herrity's ads have charged Moore with voting against several major freeways important to Fairfax commuters; Moore's ads have criticized Herrity for the county's rapid growth.

In the most recent Post survey, Moore retained 85 percent of her earlier supporters, while 74 percent of Herrity's backers from the first poll continued to support him.

In addition, 42 percent of those who identified themselves as undecided in the first poll now say they support Moore. Thirty percent of the original undecideds favored Herrity in the most recent poll, while the remainder had not made up their minds.

The two surveys also showed that Moore has been more successful in attracting fellow Democrats than Herrity has been in wooing those who identified themselves as Republicans.

Moore's support among fellow Democrats increased from 60 percent in September to 76 percent in the most recent poll. Herrity's support among Republicans increased from 47 percent in September to 55 percent last week.

Among those who voted for Herrity when he last ran in 1983, Moore increased her support. While Herrity still leads by 51 to 40 among his 1983 supporters, Moore has gained 6 points in the past month, according to the polls.