An Annandale engineer declared yesterday that many of Northern Virginia's leading Republicans support his bid to fill a vacant seat in the Virginia House of Delegates.

Louis L. Guy Jr., one of five Republicans vying for the party's nomination, said that the endorsements give him the "best chance of winning" the 39th House District seat in Fairfax County that is being vacated by Democrat Vivian E. Watts.

Both political parties plan to select a candidate Monday, probably in "firehouse primaries," in which voters cast secret ballots for a nominee. The election tentatively is scheduled for Jan. 14.

"I just don't think they my opponents would have the chance of winning a general election that I would have," said Guy, 47, an immediate past president of the Upper Occoquan Sewage Authority who has been active in engineering associations and civic groups dealing with water and the environment.

GOP lawmakers supporting Guy are Sens. Wiley F. Mitchell Jr. of Alexandria and John W. Russell of Fairfax, as well as Dels. Robert T. Andrews, Vincent F. Callaghan, James H. Dillard II, Frank Medico and Stephen E. Gordy, all of Fairfax. Harry J. Parrish of Prince William also was listed as backing Guy.

Fairfax County Board Chairman John F. Herrity announced yesterday that he supports Guy, as did County Board member Thomas M. Davis III. Both are Republicans.

Although the endorsements appeared to boost Guy's candidacy, party officials refused to call him a front-runner, and cautioned that the endorsements may not necessarily translate into support from the local party members who choose a nominee.

Republican leaders said privately, however, that they are eager to avoid a repeat candidacy by Robert E. (Lee) Murphy, a merchant who lost by 26 percentage points to Watts in their race for delegate Nov. 5. Murphy also lost in the Republican primary during the summer, but became the GOP nominee for delegate after the party's first choice dropped out. He has expressed interest in running for the seat again.

In addition to Guy and Murphy, candidates seeking the GOP nomination include Robert Dively Jr., P.J. Kirk and Robert Brostrum, all active party members in the district, according to Joanne McMurray, Republican chairman in the 39th District.

On the Democratic side, Fairfax Democratic Party Chairman Pat Watt said yesterday that there are four candidates for the party nomination.

They are James McConville, a civic activist and lawyer; Jane King, a professional consumer advocate; Ernest A. Lotito, a public relations executive and former Washington Post reporter, and Alan Mayer, a county park authority member and retired federal employe.

No Democratic front-runner has emerged, party officials said.