Under pressure from television and radio advertisements attacking her record, Fairfax County Supervisor Audrey Moore (D-Annandale) is shifting to the defensive as the race for chairman of the Board of Supervisors enters its final two weeks.

Moore, who is challenging Republican Chairman John F. Herrity in the Nov. 3 election, had an early lead in the polls and has been considered the front-runner since she announced her candidacy in April. But in recent weeks -- especially since the well-financed Herrity began a media blitz Oct. 5 to portray Moore as an enemy of better transportation -- Moore has been forced to defend her votes as never before.

In recent days, Moore has:Changed her basic campaign speech to assail Herrity's ads and insist that she voted for road projects that Herrity suggests she opposed. Issued a pamphlet, apparently in response to Herrity's literature, defending her road votes and referring to Herrity as the "developers' candidate." Fielded more and sharper questions during public appearances about her transportation record, including one from a man last week who asked Moore and Herrity to comment on Moore's record.

Moore's recent posture represents a striking reversal from just a month ago, when in a debate on WMAL (AM 63) radio she put Herrity on the defensive for having supported the county's rapid development without providing the necessary road system. Moore has been the county's leading opponent of rapid growth since she was elected to the board in 1971.

And there are strong indications that the coming two weeks will bring even sharper attacks from Herrity, a 12-year incumbent. "Fifteen days is an eternity in politics," said Michael Murphy of Murphy & Castellanos, a Republican advertising and consulting firm working for Herrity. "It's going to be the longest 15 days in Audrey Moore's life."

Janice Spector, a Moore campaign spokesman, acknowledged yesterday that "perhaps because of {Herrity's} ads the questions {from reporters} have been more pointed and the responses have been more pointed." She added, however, "I don't think it's a defensive posture at all." Herrity's negative broadcasts are a sign of a "desperate" candidate lagging in the polls, Spector said. "What else can he do but go negative?"

Moore, who lacks the financial backing of business leaders and developers, has not aired television ads. It is not clear whether she will respond directly to Herrity's commercials, but she has promised that she will soon air a spot featuring former Virginia governor Charles S. Robb, a popular Democrat who backs her.

The truth about Moore's transportation record is somewhere between the two candidates' assertions. Most road proposals go before the county board many times, and while Moore generally has voted for such projects as the Springfield Bypass and the Dulles Toll Road, she has cast several votes against specific funding mechanisms and road alignments.

Herrity and Moore opposed two important road projects in the mid-1970s, but Herrity generally has supported transportation improvements since then. He has charged that Moore's record reflects a lack of leadership. "Let's get serious about leadership," Herrity said in an appearance with Moore before more than 700 members of the Northern Virginia Building Industry Association last week. "If you're continually moving from one side to another, nobody knows where you're going {or} where you stand . . . . "

It has been an axiom of Fairfax politics this year that the candidate who is able to control the agenda will win. For Herrity, that means focusing the debate on transportation, particularly Moore's mixed record on road projects. For Moore, it means training attention on Herrity's record on rapid development, which he has generally favored.

Many political analysts, particularly Democrats, say Herrity has been helped by a group of county business leaders and developers that has launched a simultaneous television ad campaign designed to focus attention on the transportation issue. The commercials are the work of the Northern Virginia Transportation Alliance, a group that says it is bipartisan but is dominated by many of Herrity's most influential supporters.

Herrity's most recent TV ad has run in high-profile slots such as the first game of the World Series Saturday. Known to Herrity strategists as the "no-no-no ad," it proclaims that Moore voted "no" on the Dulles Toll Road, "no" on completing I-66 and "no" on the Springfield Bypass.

Democrats have protested that the ads distort Moore's record and leave the impression that Moore opposed all those roads. Herrity's aides have responded that the ads do not say that Moore "opposed" the roads -- only that she voted against them. Herrity, undeterred, last week organized high school students to stand at rush hour along several busy roads carrying signs proclaiming "Audrey Moore refused to support the widening of this road . . . . Reelect Herrity Chairman."