Virginia Lt. Gov. Charles S. Robb warned a gathering of Fairfax County Democratic candidates and their supporters last night that the party's organizational apparatus "will be tested as never before" in the Nov. 6 state election.

Robb's remarks came at a campaign kickoff rally at the Annandale home of retiring state Sen. Omar L. Hirst (D-Fairfax). Nearly all of the county's 26 Democratic candidates for legislative and local offices showed up to mingle with a small group of party workers and well-wishers.

Robb said that Republicans have targeted Northern Virginia -- and Fairfax County, especially -- as a place where they have their best hope of making political gains.

"The election outcome will depend on straight party work this time," Robb declared, adding that there is "no big emotional issue" to lure voters to the polls.

Although the issues of abortion, the Equal Rights Amendment, gun control and school busing have been hot compaign topics in the past, they are not expected to dominate this year's races.

Emilie Miller, the county's Democratic committee chairman, said a recent poll of Fairfax voters showed that funding of Metro is the top issue. Voters, Miller said, seem to want candidates who can look at many issues rather than a single subject.

The poll, according to Democrats, also refutes suggestions by some Republicans that candidates who support the E.R.A. and favor pro-choice in the abortion controversy would be hurt election day.

Northern Virginia's two Democratic congressmen, Reps. Herbert E. Harris and Joseph L. Fisher, showed up for the campaign kickoff, as did Andrew Miller, the party's unsuccessful U.S. Senate candidate last year, and state Democratic chairman Richard Davis.

Davis, according to Del. Gladys Keating (D-Fairfax), has been "up in Northern Virginia so often these days, he ought to be taking out citizenship papers."

David Temple, the only black candidate in the county's two contests for House of Delegates seats, said he is frequently asked about his chances of winning since Fairfax has so few black voters.

"But I think the public is interested in competence," Temple said. "I'm not running as a black candidate but as a candidate who is black."