With 12 days until the election, supporters of the $74.8 million school bond referendum in Fairfax County are aiming their campaign at the growing western part of the county, where new residents have accounted for record numbers of voter registrants this year.

The referendum is considered one of the most vulnerable in the county's history because of criticism of some of the projects included in the referendum from some of the county's most influential civic organizations. New voters in the county could "make the difference" in the Nov. 6 election, according to John F. Herrity, chairman of the Board of Supervisors.

"These people constitute a significant increase in the the support base of the school bond referendum," Herrity said. "First of all, I think they'll go out and vote because they've registered. And secondly, I assume they will vote for the school bond referendum because they are in the areas where the schools are overcrowded."

The bond referendum would finance the construction of five schools, including the new Braddock High School in the Pohick area, where fast-paced development has resulted in overpopulated schools.

Suzanne Paciulli, chairman of the school bond campaign, said she is planning a last-minute mailing and intense canvassing of the area over the next 10 days.

Preliminary reports from the Fairfax County Board of Registrars show that about 79,000 new voters registered in the county between January and the Oct. 6 deadline, compared with 36,000 for the same period four years ago. The outlying districts of Springfield, Centreville and Dranesville account for about half of the new registration, or 35,253 votes.

Still, some county officials are not convinced that the effort being focused on voters in the western part of the county will guarantee passage for the school bond. "In various pockets of the county, there is still some resentment against the School Board over last year's boundary changes and school closings," said Supervisor Thomas M. Davis, (R-Mason).

Also, residents who traditionally vote in local elections are likely to be influenced by the opposition of the Fairfax Federation of Citizens Associations and by the serious reservations of the League of Women Voters, said Supervisor Audrey Moore, (D-Annandale).

Even so, some county officials said the presidential race makes the outcome of the school bond referendum unpredictable. The national election will bring a turnout of voters who are less informed on local issues, but likely to vote for any school referendum. At the same time, the high turnout of voters intent on reelecting Ronald Reagan, who carried the state by 237,000 votes in 1980, may be less likely to support proposals that call for increased government spending, county officials said.

So far, most of the financial support for the school bond campaign comes from businesses, many of them high-tech and construction firms. Of the $8,480 raised in the last two months, $4,950 was provided by construction, real estate and engineering firms.

"It makes sense to go after the business community," Paciulli said, partly because schools attract home buyers and quality employes into an area. The bond referendum has received the endorsement of the Fairfax County Chamber of Commerce and the Northern Virginia Builders Association.

The bond is also strongly supported by the county's Parent-Teachers Association, which has been promoting it through a grass-roots campaign. The Fairfax County Council of PTAs' leadership endorsed the bond last month, and nine local chapters have contributed $375 to Paciulli's committee. "That's a lot of cupcakes to be sold," she said.