No official vacancy in the Arlington commonwealth's attorney's post had opened, but the two Democrats vying for the job were well into their campaigns last week when they met for a debate.

Henry E. Hudson, who was confirmed earlier this month by the Senate to be the new U.S. attorney for the Eastern District of Virginia, was scheduled to resign as Arlington's prosecutor last night. The county's four Circuit Court judges are expected to name his temporary successor this afternoon.

Most lawyers say they believe the judges will select Helen Fahey, Hudson's top assistant and a 10-year veteran of the office, to fill the job until the Nov. 4 election.

But, because county Republicans do not expect to field anyone for the post, the winner of the Democrats' July 14 "firehouse primary" (a single, as yet undetermined, polling place and abbreviated voting hours) most likely will be the new Arlington prosecutor. He or she will hold the office for the one year remaining in Hudson's four-year term.

Fahey is facing Brendan Feeley, a former assistant prosecutor who held a post comparable to Fahey's during one of the three years he served in the office in the mid-1970s. He has had a private criminal law practice since then.

The two are engaged in a hard-fought contest for the $55,000-a-year job, a match in which experience has emerged as the chief issue, as it was last week when the two faced off for the first time in a lively debate before the Arlington Young Democrats. They are scheduled to debate again today before the Arlington Bar Association.

Fahey emphasized her 10 years in the job in "one of the most efficient and effective prosecution offices in the state," handling about 1,000 felony cases and 75 jury trials in Circuit Court and 5,000 cases in the lower General District Court, which hears mainly misdemeanors.

Feeley said he did not remember the exact number of felony cases he has had, but "it would be fair to say I've represented several hundred people in the course of my legal career."

Feeley said his "broad and varied" background as a prosecutor and a defense attorney would give him a better perspective as the top prosecutor since he could anticipate and counter defense arguments. He also criticized long tenures as assistant prosecutors, arguing that the job is "one dimensional." After three years of handling cases that are essentially repetitive the job becomes "boring and redundant," he said.

"In my three years, I was assigned to handle 21 robberies, 17 burglaries and an assorted number of grand larcenies that involved the same elements of law . . . [It] is redundant. That's why so many assistant [prosecutors] move on," he said.

In response to a question about why he wants the top job if he found the experience so redundant, Feeley said, "The Commonwealth's Attorney, in distinction from the assistants, has the opportunity to set policy, work with the community, the police department, other prosecutors. It's a different job from an assistant."

Fahey said she has "not found the job to be boring or repetitive," adding that no two cases are really the same.

"It's a prosecutor's job," Fahey said. "It is not a civil attorney's job. It is not a defense attorney's job . . . [Being a defense attorney] gives you a different perspective, but it's not a perspective a prosecutor should have. A prosecutor has to have one main concern, and that's the protection of the citizens of Arlington County. Protecting the rights of the defendant in criminal cases is the function of defense attorneys."

Feeley, in a remark aimed at Fahey's five-month residency in the county, pointed to his lengthy work in community and civic affairs and his efforts to help other Democrats get elected. He also noted that he ran against the popular Hudson, a Republican, in 1983.

"It is important . . . for one who is receiving the great benefit of this office to have contributed to the community. I don't just mean in employment-related activities," Feeley said.

In response, Fahey said, "I have served Arlington County for the last 10 years. I know its citizens, I know the people who have been victimized. I even know the defendants. I know the police department, the sheriff, the fire department, the merchants, the people in the banks. That's what I have been doing for the past 10 years -- I have been serving Arlington County."

Both candidates pledged a vigorous effort to recruit minorities for the office, which currently has none. Fahey said past efforts to do so were unsuccessful because the low starting salary of $24,121 was not competitive with the private market where there are many higher-paying opportunities for minorities.

Feeley said that, if elected, he wants to inaugurate a "Crime Solvers" program in which the private sector would offer cash rewards for tips from the public that help solve cases. Arlington is the only area jurisdiction that does not have such a program.

Fahey said Arlington has its own version of that program, but that reward money comes from the police department's budget.

Police spokesman Tom Bell said earlier this week that the department opposes the "Crime Solvers" program because it feels that the acceptance of private funds gives businesses and citizens too much control over the department, allowing the private sector to decide which cases merit rewards and how much. He added that it also limits television coverage because the program is run in cooperation with WJLA, Channel 7. Feeley said he would meet with Police Chief William K. Stover to "try to persuade him of the usefulness of the program."