IF NORTHERN VIRGINIA'S envoys to Richmond don't come to their senses, they may come home penniless at session's end -- even though their very raison d'etre this year is to bring home some money for Metro. Slowly and cleverly, they're getting fleeced by the powers in the legislature -- some of whom don't much care what happens to any tax bill, not to mention one that might help people in this region. It's a sophisticated shell game in which two attractive tax proposals are dangled before the Northern Virginians and then tucked away. Just when everybody's pointing in one direction, up pops a proposal somewhere else.

It all began with Gov. John N. Dalton proposing a 4 percent tax on gasoline, for highways as well as mass transit. But that one never even got off the drawing board. He then retooled it into a 4-cents-a-gallon plan -- which the Northern Virginia delegation promptly embraced. Enter Archibald A. Campbell, chairman of the House Finance Committee, who couldn't care less about Gov. Dalton's measure; by a 17-to-2 vote, he wins committee approval for a different tax bill -- a 4 percent tax on gas purchases in Northern Virginia only.

Now, either one of these proposals would be of considerable assistance to Metro, though Gov. Dalton's balanced highway-transit measure would formally and properly include the state as a contributor to a subway system that Virginia joined with Maryland and the District to create in the first place. But by slipping the regional tax measure ahead of the statewide tax bill on the schedule in the House, Chairman Campbell may well have succeeded in killing the governor's plan. If so, the danger is that the regional tax will die while Northern Virginia's delegation argues over how or whether it should apply.

At the very least, the two bills should be considered together on the House floor, with a vote first on the statewide, highway-transit gas tax. Then if that fails, a regional tax measure -- with some language spelling out the state's responsibility to provide stable financing for Metro -- should be enacted. Either way, it is critical this year that Virginia settle on a plan that will meet federal requirements for financing the subway system. That is why Northern Virginia's lawmakers can't afford any more intramural, politically suicidal bickering while the rest of the legislature is picking their pockets.