"ONE SHOULD accept the reality of the selection process," Jervis S. Finney said the other day when he announce his resignation as U.S. Attorney for Maryland, as of Jan. 31. The "reality" is simple: Mr. Finney is a Republican, and the Carter administration and Sen. Paul Sarbanes (D-Md.) want to put a Democrat in the job. Mr. Finney had resisted the pressures until the end of the Mandel trial. But he has now decided to go peaceably - unlike holdover prosecutors in New Jersey and Detroit, who had to be pushed out and made a fuss.

The partisan guard-changing is normal. Despite President Carter's campaign talk about filling such posts strictly on merit, there was never a prayer that his fellow Democrats, especially in the Senate, would give up the patronage powers that they had just regained after an eight-year drought. More important, the traditional process does not necessarily produce poor appointments. Mr. Finney was chosen through that process, after all. So was his predecessor, Democrat Stephen Sachs. All three have been able, independent prosecutors, notably dogged and nonpartisan in pursuing corruption in high places - and notably careful about keeping their intricate, intensely controversial investigations within proper bounds. One result is the rather astounding list of top officials, headed by a Vice President and a governor, who have been brought to judgement in Baltimore's federal court. Equally important, the U.S. Attorney's office has gained such a fine reputation that the outcomes of these cases have been generally acepted as just.

Obviously antoher tradition also exists here, a tradition of tough, high-quality law enforcement that has become thoroughly bipartisan. This is the tradition that President Carter and Sen. Sarbanes should observe most carefully in choosing Mr. Finney's successor. The next U.S. Attorney for Maryland may redirect the office somewhat; indeed, he or she will probably have to, as its current series of public-corruption cases draws to an end and some members of its outstanding professional staff move on. But that just underlines the need to pick a prosecutor who - no matter how good a Democrat - can put political ambitions and allegiances aside and do the kind of law-enforcement job that the public has come to expect.