FOR MORE THAN a generation, a unique federal/state/local partnership has somehow blended various amounts of politics, money and crisis psychology to create and construct a subway system for the national capital region. But today, the final miles of a proposed 103-mile network -- including the most vital in-city link -- are yet to be built; ridership patterns are not what they once were predicted to be; and financial commitments to cover what have turned out to be enormous escalations in the tab for finishing the job are anything but solid. It won't do to rely on exhortations in the name of historic duty, cast-in-stone construction schedules or old financial formulas, either. Right now the money isn't on the table. Doubts are. Is it wise -- or even possible -- to go the last miles?

Not as envisioned. The Reagan administration has moved to cut off all federal money for Metro subway construction -- and that's been 80 percent of the financing. Congress isn't likely to be that drastic, but even the bipartisan congressional coalition working for continuing federal help is prepared for more modest math this time around. Are the state and local partners -- you can make that read taxpayers in Maryland, Virginia and the District -- prepared to make up the difference? If so, how firm is the price tag?

In 1979, when Congress was drafting the Stark- Harris authorization for Metro, $1.7 billion more was supposed to be enough to do the job. Today, Metro estimates it will need another $2 billion to finish up. There's good reason to believe that then and before -- downtown, in the federal government and in Congress -- there was much winking all around as these projections were cranked out. Inflation, delays in providing the federal money, legal disputes and route realignments all did contribute to rising costs. But not to the tune that has been sung.

Thanks to a study completed by the Federal City Council, there are new and better numbers to work with now. Besides the cost of completing the 103 miles as proposed, the study cites costs of rehabilitation and replacement to keep buses and subway cars rolling, as well as operating costs over the next 15 years. Even if the federal government continues as a contributing partner, the formula could be changed and the state/local share would mount correspondingly.

Every survey in the past has shown that the taxpayers of this region strongly support completion of the subway system -- and in fact this has been the basis for a "political" construction pattern aimed at keeping all jurisdictions involved at every stage: a bit here for D.C., there for Maryland and there for Virginia. But now those jurisdictions that have waited the longest while paying their shares are staring at each other and demanding a share of whatever money is spent.

More postponements may be necessary, along with increases in the state and local shares. Still, completion of the system should proceed. Though commuter patterns have changed and Metro won't work all the environmental, energy and mass transportation wonders that its early enthusiasts (including ourselves) liked to ascribe to it, the subway has been -- and will be -- an extraordinary asset for the inner core of the region. Clearly it has helped to reverse the decline that seemed to be under way in the years before it came into operation, helped to hold the city together and been an incentive to growth. The people who live here have paid willingly and on time to finish the project. That faith should never be blind, but neither should it be destroyed outright.

What next? In seeking reasonable assistance from the federal government to complete the subway system, Metro and the state and local governments must make their cases with much harder numbers than they have in the past. The Federal City Council study points the way; no more shrugging off every exploding cost projection as the work of inflation. If revised construction schedules are necessary to adjust to lower appropriations, Metro should be able to say what's feasible, what revisions might accelerate the Green Line, which will make such a difference to the poorest neighborhoods -- and what the cost will be of too much delay.

At the same time, the participating governments should resume in earnest their on-again, off-again, complex search for dedicated sources of revenue. If, as it seems, a single regional tax is out of the question for a heap of political reasons, how about a combination of various taxes in the region -- some more solid indication that Greater Washington remains financially and politically prepared to assume its obligations not just as a subway builder but as the operator of the system? It's called facing up to reality -- and in these hard financial times, it demands new political courage and understanding.