SINCE WHEN DID every state senator and delegate from Northern Virginia agree to stick together to the finish for one legislative purpose in Richmond? Answer: Tuesday--and don't let any of them forget it.

The reason for this sudden outbreak of political togetherness in a 29-member delegation that used to find 30 ways to split a strategy is spelled M-e-t-r-o. Members of the 21-delegate contingent in the House and of the eight-member Senate team have warned that they will oppose any tax increase for state highways that is not tied to increased money for the region's mass transit system. Blackmail? Not if you believe highways and mass transit are both part of the state's transportation system--because both need help, and that means money.

Time was when governors and legislators from elsewhere in the state looked upon appropriations to Northern Virginia as foreign aid. But sophistication and governors who get around have made great strides for a balanced transportation system in the state--and now is no time to stop. Gov.-elect Charles S. Robb has already noted that the highway department may face disaster without new revenues --make that read higher gasoline taxes--and a move in that direction is expected.

But another move--the one that would be unacceptable to Northern Virginia--would be to cut state money for Metro and use it for roads. That is what has the delegation from this region fired up and ready to do battle--and it should, in the name of fairness and sound statewide policy.