Arlington County Democrats nominated Helen Fahey yesterday for the top county prosecutor's post in the November election, giving her a decisive victory over a former assistant prosecutor who emphasized his party credentials.

Fahey, who was appointed to the job unanimously in May by the county's four Circuit Court judges, defeated Brendan Feeley, an assistant in the prosecutor's office from 1972 to 1975, by 2,465 to 1,869.

Yesterday's victory virtually guarantees Fahey's election to the office of commonwealth's attorney in November because county Republicans say they do not expect to field a candidate.

If elected, she will serve the remaining year of the four-year term of former commonwealth's attorney Henry Hudson, a Republican who resigned to become the federal prosecutor for eastern Virginia.

Fahey said she thought that she won "because we were able to reach out to a large number of Democrats who do not usually come to these elections . A great many people were very much interested in the issue of experience."

The 4,334 Democrats who cast ballots yesterday at Thomas Jefferson Intermediate School, the sole polling place, were more than twice the number predicted by party leaders.

The campaign was unusually divisive, with the camps adamant that their candidate had better qualifications for the $55,000-a-year job. Feeley had backing from the largest number of party members who are experienced in running and winning campaigns.

Feeley, 38, who ran for the post in 1983 against Hudson, emphasized his work in Democratic campaigns and his community activities. An assistant prosecutor from 1972 to 1975, Feeley has been in private practice since then, handling primarily civil and misdemeanor criminal cases. He said his work as a prosecutor and defense lawyer would make him a better commonwealth's attorney because he could anticipate defense strategies.

Fahey, 44, focused on her 10 years in the prosecutor's office, three as Hudson's chief deputy. She moved to Arlington in January from Fairfax County, where she had been active in community and school affairs.

Fahey's campaign emphasized her record of prosecuting more than 5,000 cases, including 1,000 felonies. The chief prosecutor of rape cases, she served as the counselor to the county's rape victim companion program, and she has coordinated the office's attack on child abusers and drunk drivers.

Fahey, who received the endorsement of three statewide legal associations and many lawyers, was also backed by Robert F. Horan Jr. and John E. Kloch, the top prosecutors in Fairfax and Alexandria, respectively, who said they never before had endorsed a candidate for a local office in another jurisdiction.