THIS MOST SIGNIFICANT policy change affecting the District of Columbia's criminal justice system since court reform almost a decade ago is the issue of whether the U.S. attorney's office here should be dismantled to make way for a local prosecutor.

Not surprisingly, this has also become the most vigorously debated subject among lawyers, judges, prosecutors and students of the local courts.

The U.S. attorney's office here has always been a special place and it hurts some people to talek about breaking it up. Unlike any other federal prosecutor's office in the nation, its lawyers prosecute virtually all purely local crimes, such as murder and robbery, as well as federal offenses, such as bribery and major narcotics violations. It has long been a high-visibility office, with a nationwide reputation for achievement and an impressive collection of alumni.

The D.C. corporation counsel's office is the lawyer to the city government, both in civil and in some criminal matters, such as juvenile crime, and violations of the city code, such as housing regulations. That office has traditionally suffered from a poor reputation in legal circles.

D.C. Mayor Marion Barry and other city officials, eager to push the city toward full home rule, say the time has come for the District to fully manage local affairs. And that, they say, includes criminal justice.

The federal prosecutors, and particularly former U.S. attorney Earl J. Silbert, strongly protest that such a move would fragment law enforcement efforts here. They tick off a long list of institutional reasons why a local prosecutor couldn't function in a federal city.

Whatever policy-makers may say publicly about the subject, in private they bring up other concerns - a nagging lack of confidence in the city's government and a troubling notion that race would be an overriding factor in hiring and promotions in any local prosecutor's office.

Before any transfer of authority could be made, Congress would have to enact legislation approving the change. A draft plan for shifting the responsibility, reportedly based on a model of a state attorney general's office, is being put on paper in the corporation's counsel's office. Officials said the first draft of a legislative package is expected to be released by the end of this month.

The home-rule argument appears tough to beat. In a town that craves self-control, talking against home rule is like taking a stand against motherhood.

If the District government is truly going to grow up, then it should take on the responsibility for law enforcement through the prosecutor's office - a purely local function in any other city with a population close to 1 million people.

"It seems right if you're wedded to the notion of self-government...that the people of the city would have some say in the selection of the lawyer who is responsible for the prosecution of major local crimes," said former D.C. Corporation Counsel John R. Risher Jr.

Former U.S. Attorney Silbert, now practice but nevertheless a formidable opponent of the switch, made his stand clear in a long letter to President Carter when he left the prosecutor's job in June.

For one thing, Silbert argued in the letter, a "significant portion" of the crime in the District is not purely local, but involves witnesses and victims from outside the city. The U.S. attorney's office can reach out, for example, to the FBI or the U.S. Drug Enforcement Administration for investigative help, while a local prosecutor would have limited resources.

The only valid test for the transfer proposal, Silbert said, is its "effect on the quality and integrity of justice" here. For many reasons, Silbert said, the proposal fails. Schools and hospitals may be the stuff for home rule in Washington, but not law enforcement, Silbert wrote to Carter.

Others argue, however, that if the city acts cautiously enough in moving toward accepting authority for local prosecutions, the institutional quirks that Silbert warns about can be dealt with through legislation. After all, the states seem to manage, and surely they have their share of out-of-state crime victims and witnesses. Anyway, if Silbert's argument stands that this is the nation's capital and thus it deserves some special attention, it will get it no matter who is running the prosecutor's office.

There may be some truth - and a bit of smugness - to another argument by lawyers for the federal government that the local prosecutor would not be able to draw the high caliber of young recruits who now line up for jobs at the U.S. attorney's office here. The prestige of the federal office, they say, would be replaced by the stigma of incompetence that has long plagued the corporation counsel's office.

It is hard to accept, however, that without the magic of the federal presence good young lawyers, ready for courtroom experience, would not come to the local prosecutor's office. As to the reputation of the corporation counsel's office, the city would do well to consider setting up the new prosecutor's office independently of the corporation counsel's office.

The "bedrock problem," as one lawyer put it, the issue that everybody wants to dodge, is lack of faith in the city government. The federal prosecutors are justifiably proud of what they have accomplished here, and they are afraid that it could fall apart once the system became local.

There is a legitimate concern among the opponents that the prosecutor's office might be exposed to political patronage once it came under control of the local government.

There is also a clear, although often-unspoken sense among them that, in order to survive politically, the local prosecutor's hiring policies would be directed by race.

It was tough enough to get black lawyers to enter the competition for the U.S. attorney's job, following Silbert's resignation. So, how would young black lawyers, with options to work in the city's best firms, be persuaded to join the local prosecutor's office?

The city's response is that, if the lawyers want litigation experience, if they want to get into a courtroom and try a case, they would come to the local prosecutor's office, just as they have gone for years to the federal prosecutor's office.

How politics and race would affect the new local prosecutor - or attorney general's office if that were the case - might not be visible for several years after the change occurred.

If the District government wants to meet those concerns - and it should - it should focus attention on the procedures for selecting the local prosecutor. As the plan stands now, the local prosecutor would be appointed by the mayor and subject to confirmation by the D.C. City Council. With appropriate safeguards, that plan, as opposed to an elected prosecutor, might keep politicking to a tolerable level.

"I'm as afraid of it as anybody else," one defense lawyer said, "but there's a principal involved here" - home rule.

One crucial prerequisite to making the transfer work is money. It costs about $7 million, to operate the local division of the U.S. attorney's office, paid entirely by the Department of Justice. And observers who have had some unpleasant experiences with local budget cuts have their doubts that the city would raise the cash to keep salaries comparable and maintain the current level of resources in the federal prosecutor's office.

About 80 assistant U.S. attorneys now prosecute local criminal cases in Washington and another 90 lawyers staff the D.C. Corporation Counsel's civil and criminal divisions. Salaries in the two offices are comparable to attorneys right out of law school - about $19,000 annually - but federal wages are higher as experience and supervisory duties increase.

Supporters of the transfer insist that, if the plan were to go through, the money would come. They remind skeptics that District residents paid 75 percent of the federal prosecutor's budget until 1975 and they can do it again. And they don't rule out the possibility of a special allocation from the federal government.

The worst thing the city government could do over all would be to sit back and presume that the momentum over home rule will carry the issue through local opposition - particularly from Silbert and in part from the judges of the D.C. Superior Court. They too, in a surprising display of opinion, have expressed serious concerns about the transfer, and particularly about Barry's insistence that he should appoint the local judges, subject to confirmation by the D.C. City Council.

Couching the issue in terms of self-government is politically smart. But, practically speaking, District officials had better be well fortified with details and the promise of cash if they want to dispose of what Barry sees as one of the lingering vestiges of the "Last Colony."

OBITER: Three of 12 partners in vom Baur, Coburn, Simmons and Turtle in Washington have given notice that they intend to leave the firm. Departing are Richard C. Johnson, Bruce Shirk and James McHale. Two more partners, reportedly planning to leave vom Baur's California office could not be reached for comment...James R. Trimm of Potomac is the new president-elect of the Bar Association of Montgomery County Trimm will take over for current president Paul Mannes in June of 1980...Robin A. Smith, assistant D.C. Bar counsel will join the American Bar Association's Center for Professional Discipline in September...Alexandria attorney Kenneth Foran had been appointed to the Virginia State Board of Accountancy, which gives the state exam for certified public accountants...and Truman A. Morrison III, formerly chief of the training division of the city's Public Defender Service, will be sworn in on Sept. 7 as an associate judge of the D.C. Superior Court.