An Arlington police officer who wants to run for sheriff filed a lawsuit yesterday challenging a county law that requires him to resign from the force when he announces his candidacy.

Patrolman John E. Baber, a 14-year veteran of the police force, asked the Arlington Circuit Court to block enforcement of the ordinance, nicknamed "Arlington's mini-Hatch Act" after the federal law that bars civil service employes from engaging in partisan politics.

Unlike Alexandria and Fairfax County, where employes can take unpaid leaves for such campaigns, Arlington requires its employes to resign. Baber, 45, wants to take an unpaid leave of absence while he pursues the Republican Party's backing of his independent campaign for sheriff.

"On its very face, it's the county law unfair," Baber said in an interview. "It's not saying I can't run . It says if I do, I've sacrificed everything I've worked for for nearly 15 years."

If he resigns, Baber said, he will lose his salary and medical benefits. If he does not get the GOP endorsement, he will have to seek "another source of income . . . but if I get a leave, everything would pick up where I left off after the election ."

Kevin T. Fitzmorris, his attorney, called the Arlington law unconstitutional and said there has been a number of recent court decisions, striking down such regulations as a violation of the rights government workers have to political expression and equal protection under the law.

The Arlington County Board, which is empowered to give Baber a waiver that would allow him to take an unpaid leave of absence, has requested a staff study of the issue. The study is expected to be completed before the board's next meeting, which comes a few days before the April 15 filing deadline for the Arlington Republican Party's canvass.

Fitzmorris said any board action at its April 9 meeting would leave insufficient time for Baber to marshall his campaign supporters. Baber faces at least two other candidates also seeking the GOP endorsement for their independent campaigns.