It has been a spring of discontent for Arlington's Republican and Democratic parties.

The Republicans are in the midst of a contest for chairman of the county committee in which a self-described moderate is challenging a heavily favored conservative.

The Democrats, while united on other campaigns, are feuding over who should be the party's nominee to succeed county prosecutor Henry E. Hudson, a Republican who has been nominated to become the U.S. attorney in Alexandria.

In recent years, political infighting in Arlington has mostly been confined to the GOP. But this year the Democrats are embroiled in such an impassioned battle over the prosecutor's job that it erupted last week into uncharacteristic acrimony at a meeting of the county committee.

The Democrats' feud takes on more significance this year because the person they nominate this spring will most likely be the next commonwealth's attorney. The Republicans say they do not expect to field a candidate, and the chances of an independent running are seen as remote.

The Democrats have split into camps supporting Helen Fahey, deputy county prosecutor and a 10-year veteran of the office, and Brendan Feeley, an assistant prosecutor for three years in the mid-1970s who has a private criminal law practice. He ran as the Democratic nominee against Hudson in 1983, a credential that has figured strongly in the dispute.

If Hudson is confirmed by the Senate to the federal post, as expected, the county's circuit court judges will appoint a successor to serve in his current post until a new county prosecutor is chosen in November's elections. The judges, themselves appointees of a Democrat-controlled state legislature, are expected to appoint a Democrat. Fahey is widely expected to be that Democrat.

Regardless of whom the judges appoint, Fahey and Feeley are likely to compete this spring in what could be a divisive primary battle to win the party's nomination to run this fall. The race has already brought charges from Fahey supporters that some Democrats are putting party loyalty ahead of merit.

"I'm a strong partisan Democrat, but I think I shouldn't be a blind one," said Arlington Sheriff James A. Gondles, a Fahey supporter. "I just think it's very clearly a case of experience. Were they equal, I would support Brendan because of his political background. But I don't think political background alone should qualify anyone to hold office."

"That's nonsense," Kevin Appel, a lawyer and deputy county treasurer, said in response to Gondles' general allegations of blind loyalty among Fahey supporters. "Brendan is highly qualified. If you have someone who's qualified and has experience, he deserves to get the edge over someone who may be equally qualified but hasn't worked" for the party.

Fahey said she is a lifelong Democrat, but she has not been visible in local party politics until recently. She has come under attack for what she said was a two-hour election day stint handing out literature for her boss, Hudson. Feeley, in contrast, is a member of the local party committee, which remains grateful for his willingness to run against the popular Hudson three years ago.

Feeley has won the support of a substantial number of party loyalists, the workhorses who get out the vote that has led to many overwhelming Democratic victories recently. Fahey, too, has an impressive list of backers who say they face the uphill but not insurmountable job of "spreading the word" about her -- and beating the party "machine" cranked up for Feeley.

Fahey's supporters' task was made more difficult last week when the local committee adopted a nominating procedure pushed by the Feeley faction. They selected a so-called "firehouse primary" in which voters have to come to one place instead of being able to vote in their own precincts. The process, Fahey supporters say, favors Feeley because it is more likely to bring out the many hard-core Democrats supporting him than the average voters who might find the trip outside their precinct inconvenient. The date of the primary is up in the air until Hudson resigns.

"The railroad is chug-chugging along," an angry Del. Warren G. Stambaugh, a Fahey supporter, said at the committee meeting after Feeley supporters prevailed. His charge brought similarly angry denials from Feeley backers, who countered that state electoral laws made a regular primary impossible.

The candidates, who sat quietly through the bickering, later gave their own reasons for seeking the job:

"I think this is an absolutely critical job to the safety and security of the community," said Fahey, a Catholic University law graduate. "Of the two candidates, I have considerably more trial experience, criminal experience and direct prosecution experience."

Fahey, who teaches a trial-practice course at George Mason University Law School, points to her 10 years of experience prosecuting "hundreds" of felony trials and helping Hudson run the office as chief deputy since 1983.

"I just feel very comfortable with the mission of the commonwealth's attorney . . . going after people who victimize others," said Feeley.

Feeley, a University of Virginia law school graduate, brushes off opponents' criticism that he hasn't prosecuted since 1975 and has not defended a large number of accused felons in recent years.

After three years as a prosecutor, he said, the experience became so repetitive that he entered private practice, where he deals with the same rules and procedures of criminal law that prosecutors do. His work as prosecutor and defense lawyer has given him insight into what an adversary's strategy might be, he said.

Although he accepted a number of court requests to defend accused felons early in his practice, Feeley said he has not handled many felonies recently because most serious crimes involve indigent defendants who get court-appointed lawyers. However, he said he has in the past year accepted a court-appointed murder case and has been privately retained in two felony drug cases.

The county's Republicans are facing a less raucous spring, but they are not without problems. Scott McGeary, the local committee's secretary and legal counsel, is expected to win a resounding victory for chairman of the party at its May 10 canvass, a version of a local primary. The current chairman, Helen Blackwell, a conservative, is not seeking reelection.

McGeary is being challenged by Thomas W. Dennison, a maverick who says he is attempting to rally moderates to end the conservatives' control of the party. Dennison also said he will challenge Democratic County Board Member Mary Margaret Whipple if the GOP doesn't find another candidate.