WITH THE BLESSING of the federal executive branch and hopes for congressional approval, the District of Columbia is about to take its next step toward full self-government. Under a long-awaited proposal outlined by local and federal officials, the District government would assume full control over its local criminal justice system by 1981. Though this transfer poses a serious problem for the city government, it is a change that is right. The challenge is to make the switch without diminishing the high quality of justice that the city has enjoyed under the federal-local arrangement.

That won't be easy. In the years of federal authority over criminal cases here, the office of the United States Attorney for the District of Columbia has earned a good reputation for criminal prosecutions. The Justice Department has been able to attract talented people for stints in the DC. office, many of whom have gone on to distinguished careers in private and government practice. But once an office with "national" connections goes local, it will take some doing to recruit and retain the same level of talent. Evidence of that difficulty can be found already in the D.C. Corporation Counsel's office, which prosecutes the less serious, local criminal cases and handles civil matters involving the city government.

Still, the selection of somebody at the top who will demand top-quality legal work could make a significant difference. This, according to the transfer proposal, would be the local attorney general, who would be nominated by the mayor and subject to council confirmation. Similar care should be exercised in the selection of an associate attorney general, who would control all the local criminal prosecutions now handled by the U.S. attorney's office. In addition, there could be some informal arrangements between the city office and the U.S. Justice Department for special recognition of attorneys who serve the local system with distinction for eventual positions in the federal system as well.

It is unfair, therefore, to assume categorically that the transfer of this authority to the city means inferior prosecution. The D.C. Superior Court's evolution from a small-time drunks-and-traffic operation to a highly respected DC.Equivalent of a state court is a success story, thanks not only to generally good prosectuion, but also to strong administration and the selection of some good judges. The Justice Department, which in the past has objected to the transfer, now supports the idea, and legislation is expected to go to Congress in November. It deserves careful consideration on Capitol Hill -- not with an eye toward killing it -- but with an objective of making the transfer as smooth as possible.