The defeat of moderate Republican legislator John H. Rust Jr. should help Northern Virginia Democrats pick up at least one seat in this fall's elections for the House of Delegates, party leaders said yesterday.

Some Fairfax County Democrats said the conservative wing of the Republican Party had accomplished what for the Democrats was only a dream -- defeating a popular incumbent in a heavily Republican district.

Retired Army colonel Stephen E. Gordy, who beat Rust by 15 votes after a hard-hitting campaign, moved to the top of the Democratic Party's target list, they said. The Republicans occupy eight seats in Fairfax County's 12-member House delegation.

"I do think there are a couple of seats we could pick up," said Dottie P. Schick, Fairfax Democratic Party chairman. Other Fairfax Democrats listed incumbent Republicans Robert T. Andrews, Gwendalyn Cody and Frank Medico as vulnerable.

Not so, said House Minority Leader Vincent F. Callahan Jr., an eight-term Republican legislator from McLean. "We're in excellent shape," he said. "We had some rough primaries in Fairfax but it's not going to affect the final outcome."

Rejecting an early redistricting plan, a federal court ordered the House's 100 members to run this year for special one-year terms. The court finally accepted a plan that placed all 100 legislators in individual districts -- a step that Callahan said could help Republicans add as many as six seats to their current total of 33.

"I think this is the key year; this is the first year out in single-member districts," said Callahan. "After this, incumbents will have a leg up in the future."

The Republicans' new-found advantage is expected to come in urban areas where Democrats until now have combined their strengths and run together in multimember districts.

But in Northern Virginia, Republicans and Democrats agree that most incumbent legislators still have an edge, partly because they were able to draw their own district lines.

The five Democrats who represent Arlington and Alexandria generally are thought to have among the most secure seats in the Washington suburbs.

In Fairfax County, some Democrats said they believe the GOP's three freshmen legislators are most vulnerable.

Some Republicans strategists said yesterday they believe their party can hold onto its 10 Northern Virginia seats and possibly defeat Democrat Floyd Bagley of Prince William. Bagley, a Dumfries lawyer running for the first time in a largely Republican district, will be opposed by legislative aide John Rollison.

Nora A. Squyres, a Falls Church area feminist who ran unsuccessfully last year, is hoping to unseat Cody, an opponent of the Equal Rights Amendment who was first elected last year. David L. Temple, a black educator who has also tried before, hopes to knock off first-termer Medico in a Mount Vernon area district. And Marie Ridder, wife of a newspaper magnate, is organizing a well-financed campaign against freshman incumbent Andrews in a district stretching from McLean into Loudoun County.

Callahan conceded that Rust's defeat will cost Republican votes in the general election but said the conservative bent of the Fairfax City district may prove decisive. "Internal divisions are never a plus, but we've survived them before," said Callahan.

Both parties are fielding candidates in most of the Northern Virginia races despite the strength of many incumbents. Shortly before a filing deadline Tuesday, Republicans nominated M. F. (Frank) Ruppert to run against Del. Kenneth R. Plum in a Reston-area district and Eric Mansfield to challenge one-term incumbent Del. Vivian Watts in an Annandale district. Democrat Lee Strang, twice a losing candidate for delegate, agreed to challenge Republican moderate James H. Dillard II in the Burke area.