JUST WHAT IS the White House threatening to do to the region's subway system? As of yesterday, the administration was holding up millions of dollars in grants, including $70 million that is needed almost immediately for 90 new subway cars. Why the holdup? Because the White House is pressing for a financial pledge from the local governments - one that they can't deliver.

At issue is how to cover the debt on $1 billion in construction bonds that were sold by Metro. The local jurisdictions and the federal government had been scraping up semiannual interest payments through one method or another; then, about six months ago, they tentatively agreed that the federal government would pay two-thirds and Metro the remaining third. The understanding was that there would be a period of transition to permit the area governments to work the one-third payment into their local budgets. But thanks to a number of non-happenings in Richmond and Annapolis - events that were beyond the local governments' control - there is no way right now that they can produce the "full faith and credit assurances" and payment schedules that the administration seeks.

In fact, as Metro board chairman Jerry A. Moore Jr. notes, a complete local guarantee would require, among other things, voter referendums for certain Virginia jurisdictions as well as Prince George's County. Besides, the Maryland legislature has a mandated financial study under way for consideration next year; and the Virginia legislature won't take up the question again until then, either.

That said, the administration is justified in seeking the most specific agreement possible at this point - and perhaps the participating state and local officials should initial something for the record. But the local commitment is certainly evident; not only is Metro ready now with its share of the July 1 interest payment, but it is about to approve a construction schedule as curtailed by the White House budget. So the administration should stop this money minuet and let Metro buy those subway cars on time while continuing to uphold its end of a bargain.