DURING HIS CAMPAIGN to become Prince George's county executive, Lawrence J. Hogan said he wanted minorities represented in the top ranks of the police department-and he has been trying to keep his word. For one thing, he wants to fill a vacant lieutenant colonel's job with a black officer from another law-enforcement agency. But a majority of the county council, after heaving lobbying by the police union, has found a cunning way to block the move: Yesterday, the council voted 6 to 3 to cut out the money for this job.

Council members offer several excuses for taking this preliminary action (it is still subject to another vote). If Mr. Hogan wants a high-ranking black officer, some say, he should fill the vacancy with one of the 55 blacks already serving on the 837-officer force. That may sound reasonable-but when the highest-ranking black is a sergeant and there's one black sergeant on the force-it is not the way to select someone for a top-command rank. As Police Chief John W. Rhoads said in a Washington Post article last Saturday. "You cannot always wait for promotions from within or until certain expertise is developed internally" to fill a pressing need.

Council members also say they would prefer that Mr. Hogan create a new position that would be exempt from regular hiring and promotion requirements. But this would involve voter passage of an amendment to the county charter-which couldn't take place before November 1980. Mr. Hogan was trying to fill the job by July of this year. He says that under the country merit system, the government has authority to look outside a department to fill any position that opens up if there are five or fewer people at the rank below it.

For the long run, both Mr. Hogan and Chief Rhoads favor legislation exempting several or all positions above the rank of captain, leaving those appointments to the chief. This could serve several constructive purposes, not the least of which would be to allow each chief to select a compatible top staff of policy-makers. For now, however, the county council members should rise above the misguided opposition of a union leader and allow Mr. Hogan and the chief to make their choice.