Arlington firefighters filed suit against the county government yesterday, charging that a civil service rule barring them from participating in local and state political campaigns is unconstitutional.

The suit, filed in Arlington Circuit Court, said the county is the only jurisdiction in Virginia that prohibits its police, fire and other employes from active participation in elections.

The suit, which seeks an order blocking the county's enforcement of the 31-year-old rule, was filed by three employe groups and four county workers who said they were barred under the rule from seeking election as delegates to a state Democratic Party nominating convention last year or from participating in this fall's political campaigns in Virginia. Their suit charges that the Arlington rule violates both the workers' First Amendment rights to petition government and their 14th Amendment rights to equal protection under the laws.

"The plaintiffs want the right to be politically active," said Abraham L. Swerdling, an attorney representing the Arlington Professional Firefighters Association and two American Federation of State, County and Municipal Employes (AFSCME) Council groups and the four employes.

County officials said the law was instituted in 1951 to discourage politicalization of county jobs. The law periodically has been debated by the County Board but never has been substantially changed, according to a spokesman for the county attorney's office. It is patterned after the federal Hatch Act, which prohibits federal civil service employes from engaging in certain political activities.

County officials said they had not seen the suit, which was filed late yesterday. County Manager Larry Brown, who reported for his first day at work yesterday, said he would bring the suit before the County Board at its meeting Saturday.