IN MID-JULY, the House of Representatives gave Metro a boost it desperately needed. By a stunning 261-to-125 vote, members passed a laboriously negotiated bill to provide matching funds for the transit system and thus obligate the local governments to find a reliable source of financing. But now the potential success of this important legislation is threatened by inaction in the Senate, where the bill rests with the District subcommittee headed by Sen. Thomas F. Eagleton (D-Mo.). With a little more dawdling, in fact, the necessary impact of the measure on Metro's financial fate in Annapolis and Richmond could be lost entirely.

At this point, Chairman Eagleton is talking about holding hearings on the bill in October, but barring a rare burst of senatorial speed, that's likely to mean the bill wouldn't be approved before Congress quits for the year. Enactment this year is crucial because the Maryland and Virginia legislature convene in January, and their understanding of -- and financial support for -- Metro will hinge on the federal package. In both states, the local government's authority to raise their shares of Metro financing also will depend on approvals by the state legislatures.

So instead of hesitating to move on the Metro bill, Sen. Eagleton should recognize that it is not simply a fat federal giveaway measure, but a fiscally responsible plan for completing the subway system and binding the local governments to a workable financial arrangement -- a project supported by Congress and five presidents. A sixth president, Mr. Carter, still hadn't grasped the sensible, moderate financial approach of this bill when the House members did in July; but waiting for the Office of Management and Budget to understand how such arrangements actually can save taxpayers' money over the long run is a dangerous game -- as Congress learned when Mr. Carter vetoed a District pension bill that would have cut costs and ended abuses.

In this instance, there is an even better policy reason for supporting the bill. Metro is the one new mass-transit system in the country that is on the verge of having a substantial impact on commuter habits -- gasoline consumption, air pollution and all the other justifications for good subway service. The best demonstration of commitment to these objectives right now would be prompt Senate approval of the Metro legislation.