A prosecutor and a defense attorney are battling for the Democratic nomination for Arlington County's chief prosecutor in one of the most divisive political campaigns in years.

The contest between Helen Fahey, 44, who was appointed to fill the vacant commonwealth's attorney's position temporarily, and Brendan Feeley, 38, a defense attorney, has sharply divided local Democratic officials.

The most fiercely debated issues have centered on the experience levels of the candidates and Feeley's role as a Democratic activist.

The Democrats will nominate a candidate for the fall general election Monday in a "firehouse primary" -- for pledged Democrats only -- at Thomas Jefferson Intermediate School near the intersection of South Glebe Road and Arlington Boulevard (Rte. 50). Amid predictions of a close race and expectations that fewer than 2,000 voters will turn out, each campaign is working zealously to get Democrats to the polling spot from 11 a.m. to 9 p.m. Monday.

The Democratic nominee is virtually guaranteed the $55,000-a-year commonwealth's attorney job because county Republicans say they do not expect to have a candidate in November and no independent is expected to run.

The seat became vacant when Henry Hudson was appointed U.S. attorney for the Eastern District of Virginia in May. His successor will serve the remaining year of his four-year term.

Fahey has worked in the prosecutor's office for 10 years, the past three years as Hudson's chief deputy. She was appointed to the top job in May by the county's Circuit Court judges. Feeley was an assistant commonwealth's attorney from 1972 to 1975 and since has had a private criminal and civil practice.

The candidates have engaged in heated debate as to which is more qualified to hold the job.

Fahey, pointing out her experience, said, "I have handled over 1,000 felony cases in Circuit Court in the past 10 years. My opponent's criminal experience in the last 10 years has been limited almost exclusively to misdemeanor and traffic cases."

Feeley argued that he is better qualified because he has worked as a defense attorney as well as a prosecutor: "We have an adversarial system of justice. Prosecutors and defense attorneys approach problems from contrary perspectives. It's useful to have experience with both perspectives."

Another major issue has been the Feeley campaign's emphasis on his Democratic Party connections.

Arlington Democratic Del. Warren G. Stambaugh, a lawyer supporting Fahey, said, "Aside from the turnout, it's really just a question of whether experience and ability are going to win out over political good-ol'-boyism. There's clearly no question Helen Fahey is the more experienced and qualified candidate and Brendan Feeley has the political clout."

Del. James F. Almand, an Arlington Democrat and a lawyer supporting Feeley, disputes that: "Without doubt, Brendan Feeley is eminently qualified to be Arlington's chief prosecutor because of his competence and experience trying what has been a substantial and significant number of cases as a prosecutor and a defense attorney."

Feeley, a University of Virginia law school graduate, and his supporters have focused primarily on his work for Democrats, including an uphill struggle in 1983 against the popular Hudson, a Republican who ran as an independent. Fahey said she is a lifelong Democrat and has had limited time to work for party candidates because she raised her two children alone after her divorce and spent 17 years putting herself through Catholic University's night law school. Feeley is single.

Each of the candidates expects to spend $10,000 on the campaign. They two lawyers have won endorsements from women's groups and various lawyers. The County Board's four Democrats have been neutral.

Fahey is supported by 10 past presidents of the county bar association and the immediate past presidents of three statewide legal associations. Robert F. Horan Jr. and John E. Kloch, the commonwealth's attorneys in Fairfax County and Alexandria, respectively, endorsed her in an unusual move to back a candidate for local office in another jurisdiction.