The Arlington County Board decided last night to consider modifying a law restricting the political activities of county employes, but the proposed new rules wouldn't help the man who has been trying to get them changed.

John E. Baber, a county police officer who wants to run for county sheriff, has contested a statute--the so-called "mini-Hatch Act"--requiring county employes to resign or be fired if they run for elective office. His court challenge of the law was rejected Monday by Arlington Circuit Judge Paul D. Brown.

Last night the County Board, which Baber had asked for a leave of absence while he runs for sheriff, decided to consider changing the law. The change, on which the board will hold a public hearing May 21, would allow county employes to keep their jobs while running for elective office--but only if that office is outside the county.

Baber said yesterday he plans to appeal Judge Brown's ruling to the Virginia Supreme Court and to go ahead with his independent candidacy for the sheriff's office and seek the county Republican Party's endorsement May 17. If he fails to win that endorsement, he said, he will drop out of the race.

Two other independents are seeking the Republican endorsement for the sheriff's post, now held by Democrat James A. Gondles.