THE VIRGINIA HOUSE of Delegates will vote today or tomorrow on the proposal to give local governments in Northern Virginia power to impose a sales tax to raise funds for Metro and other transportation projects. It is the most important vote to be taken in that body this year - important vote to be taken in that body this year - important not only to Northern Virginia but to the entire Washington area. If the bill becomes law, the road will be open to a sound financial plan for Metro and to solvency for the Virginia suburbs. If it fails, both Metro and the suburbs will remain in critical financial condition.

The vote on the bill is likely to be close. Although it had passed the Senate with relative ease, it was killed by the House Finance Committee, 10 to 9, on Monday. But heavy lobbying overnight by Northern Virginia legislators and other public officials resulted in a change of heart by one committee member so that the bill was reconsidered and sent to the floor of the House of Delegates by a 10-to-9 vote on Tuesday. If the bill is to pass, that lobbying effort will need to have been equally successful with other members of the House, including one or two from Northern Virginia.

The bill would authorize an additional 1 percent sales tax if the local governments concerned agree to it and if they also agree to reduce other taxes. That means the taxes on real estate and businesses, the only major sources of revenue that local governments now have, would come down. As the bill now stands, two-thirds of the $34 million or so raised by the sales tax would go to pay Metro's bills, and the rest would go to local governments.

The need for this clear. The financial demands of Metro are growing. The tax rates in Northern Virginia communities are about as high as they can be pushed - far higher than they are elsewhere in the state. The General Assembly has shown no inclination to increase the state's contribution to Metro beyond $5 million a year. And the money to meet the bills has to come from somewhere. A sales tax is not the best mechanism with which to raise it, but nothing better is in sight.

In addition to Northern Viginia's need for the money, however, there is a more fundamental question. It is whether the General Assembly is going to permit local governments in the state to run their own affairs. Raising the money to pay the costs of Metro is primarily the responibility of the local governments in Northern Virginia. But giving them the tools they need to raise that revenue is the responsibility of the General Assembly. This bill would provide one of those tools. With it, Northern Virginia will be on its way toward solving by itself its most difficult problem. Withour it, the problem will continue to grow until only the state will be able to cope with it.