Greater Washington's subway system -- the grand and often imperiled mass transportation project for the capital that has enjoyed the support of every president since Dwight Eisenhower -- is now in its deepest trouble ever. The Reagan administration's Office of Management and Budget has recommended cutting out all federal spending for completion of the system as envisioned, negotiated and brought along through years of cooperative efforts by federal, state and local governments. It's called "zeroing out" and would take effect in the next fiscal year. The result: routes patiently awaited might never be built. The biggest losers would be the residents of low-income neighborhoods who depend most on mass transit -- and who have had to wait the longest.

There's no question that under Gramm-Rudman- Hollings plenty of harsh adjustments are in the offing and that Metro faces its share. But OMB's proposal would undercut delicately negotiated agreements reached by this very administration and all the other participating governments. They already had worked out schedules for keeping subway construction as orderly and economical as possible, within strict financial limitations. That is the only fair way to treat those parts of the region that over the years have committed and pooled their state and local money for other routes while theirs had to wait.

It isn't as if this adminstration had been mindlessly tossing dollars at some high-speed plaything for the locals. Secretary of Transportation Elizabeth Dole and Ralph L. Stanley, the administration's mass transit chief, have won agreements with Metro that are considered models of sensible austerity. And if they believe in what they have accomplished so effectively for a balanced mass transportation system in the capital, they should appeal in the strongest terms for a more rational adjustment. It would be a recognition within the administration of what they have achieved on its behalf already.