STEADFASTLY UPHOLDING their right to be unreasonable and provincial, four of Prince William County's seven supervisors have struck a blow against the preservation of historically valuable land around Manassas National Battlefield Park. Unless Sen. Harry F. Byrd (I-Va.) comes to the rescue, the land is likely to be irretrievably ruined by those who would trade in Virginia's heritage for just about any tacky tourist trap or commercial complex. As things stand now, Mr. Byrd has a House-passed preservation bill bottled up in the committee; he could - and should - see to it that the measure gets proper Senate consideration.

This is the second time that the House has passed the bill, only to see it suffer at the hands of a few obstructionists. In the last Congress, Sen. William L. Scott (R-Va.) single-handedly blocked Senate passage by exercising a proxy in the closing hours of the session when he happened to be off on a junket. Now the bill is getting nowhere because Sen. Byrd has been saying that he wanted to see what the local supervisors thought. Their response has been to stall; for months, they refused even to hold a hearing on the plan. Finally, thanks to two supervisors who recognize the ntional historical significance of the preservation proposal - Kathleen Seefeldt and James J. McCoart - a hearing was held Monday on a compromise offered by the bill's prime sponsor, Rep. Herbert E. Harris (D-Va.). Then on Tuesday, the majority voted to reject the compromise.

But the decision is not up to the local supervisors, and the question is not merely whether a county should put this or that on a given site. The matter is of historical importance to the state and the nation - and it is for Congress to decide. Besides, Rep. Harris and the Interior Department have made every effort to reason with the local officials, two of whom rightly believe that a satisfactory compromise can be worked. We'd like to believe that Sen. Byrd - whose father, the late Sen. Harry F. Byrd Sr., was instrumental in the moves to preserve the land by creating the park - will not forsake the follow-up effort now.