NOT SINCE the birth 15 years ago of a new home-rule charter government for the District of Columbia has there been a more important and timely professional diagnosis of this city's finances and programs than the report and recommendations made by the Commission on Budget and Financial Priorities under the chairmanship of Alice Rivlin. It confirms and quantifies what was known -- that the District confronts an immediate fiscal crisis of serious proportions. If the city does not act quickly and dramatically to cut spending, raise revenues or both, it faces a budget deficit of at least $90 million, rising to at least $200 million in 1991 and $700 million in 1996. If the news is grim, the timing of the findings and recommendations from this first-rate commission of experts from various fields and both political parties could not be better: in less than seven weeks, Sharon Pratt Dixon -- swept into office as mayor with an overwhelming mandate to clean house at the District Building -- will now have invaluable data on which to make her pledge stick.

The commission begins its report with a warning on precisely this point -- that unless spending and revenues are balanced in a quick, orderly and constructive way, "insolvency will force deep and arbitrary cuts in public services, massive layoffs of District employees and even a potential threat to home rule." The report cites a number of causes of this predicament, including a slowdown in the city's economy; a drug epidemic with its related effects on crime, corrections, human services, education and public health, and a number of other "stresses"; the loss of middle-income residents, jobs and sales to the suburbs; an increasing concentration of the region's poor; declining federal support; and local government staffing levels much higher than those of other cities no matter how you factor in state functions and other special services.

Therein lies the core of mayor-elect Dixon's pledge to turn things around, to pare the enormous financial outlay now committed to salaries, overtime and benefits for 48,000 employees. Here too is perhaps the most dramatic recommendation of the commission: that total staff should be reduced by approximately 6,000 positions. It cites 2,100 middle-management and administrative positions that could be eliminated by cutting "excessive levels of management, too narrow spans of control, and excess staff in high-grade executive assistant, administrative assistant and deputy-type positions." Another 1,900 positions could be dropped by improving management techniques, eliminating overlap and consolidating fragmented responsibilities. And yet another 2,000 by contracting with private firms for certain services.

The beauty of this report is its remarkable specificity. It could well take the place of the management audit that Mrs. Dixon has said would be the cornerstone of her efforts to prune the bureaucratic overgrowth. In the field of human services, for example -- more than one-third of the city's budget -- the commissioners recommend restructuring, consolidation, changes in foster care and child protective services, with a greater emphasis on early intervention and ways to keep families intact; and fewer but more comprehensive clinics that will both offer better care in one place and increase reimbursements from the federal government and third-party health insurers.

There is a call for savings in the school system, too -- not by hurting what happens in the classrooms but by getting rid of classrooms not needed as well as some people not needed: a top-heavy, excessive central administration "overstaffed by as many as 800 positions." Here as in all the calls for cuts in positions, the commission is not talking about massive firings, but rather of not filling positions that are or become vacant and other shifts that can be effected within the personnel and contract provisions.

Perhaps the most startling finding has to do with the police force. Contrary to popular belief, the commission concludes that research shows no significant relationship between the number of people on the force and the rate of crime. The report recommends that the city reduce the number of authorized police positions -- not the number of officers on patrol -- by 1,605. This could be done by eliminating 1,000 new and as yet not totally filled positions authorized by the D.C. Council and Congress, as well as 527 additional uniformed and largely management and administrative jobs, and by converting 78 positions from uniformed officers to civilian employees.

In the corrections department, too, the commission recommends some ways to eliminate certain positions: house arrests, more electronic surveillance, better use of work release, halfway houses and other community supervision programs for inmates convicted of misdemeanors who now are mixed with others in costly secure confinement facilities.

As every serious candidate for public office here noted, there is no question that the federal government has shortchanged the city and that the city should have the authority to tax nonresident income earned in the city; barring this, the federal payment should be raised to reflect this loss as well as other costs that this payment is meant to help cover.

What almost every candidate for public office took pains not to note -- that certain taxes might have to be raised after all else is said and attempted -- is addressed squarely in the report: better collections of existing fees, taxation of certain activities not now taxed (such as expansion of the unincorporated business franchise tax to currently exempt businesses) and a sales tax on professional services and on mail-order and TV home-shopping purchases if a federal prohibition on such taxes is removed.

Obviously there is much to digest here and not much time to do so if a serious mobilization is to work. Not even the commission members believe that every last recommendation will be followed, universally hailed or found to be 100 percent effective as prescribed. What the city has here is a remarkable inventory that shows pockets of efficiency as well as inefficiency. As chairman Rivlin notes, "The notion that nothing works in the District of Columbia, or that everybody is a crook, is simply not true." Neither did the commission find this city in the dire straits of Philadelphia or New Orleans. The region is not without special assets and "a strong community base." The city also has fresh thinking on the way to the top executive and legislative ranks of city government. It is, Mrs. Rivlin says, "a moment when things could go right."

Where things cannot be allowed to go are those shelves on which so many studies wind up, or the deadly mined fields of political demagoguery and pettiness that kill vision and blow opportunities for substantive financial improvements. The District has been presented here with an exceptionally useful working document on which much more will be said and written. Mayor-elect Dixon, council chairman-elect John Wilson and everyone upon whom they call for help will be put to exceptional tests of political courage and administrative skills. The reviews won't be in overnight either. But the benefits of success are more than monetary. An improved image of this city around the world, in addition to right here on Capitol Hill and in the White House, would bolster the local pride as well as the financial underpinning that the District so desperately seeks to recover.