AFTER MANY lost battles in the Senate, supporters of a bill to preserve historically valuable land around Virginia's Manassas National Battlefield Park are having another go at it in Congress. This year, fortunately, the outlook has improved greatly, since 1) William L. Scott of Virginia is no longer in the Senate; 2) the other senator from Virginia then and now, Harry F. Byrd Jr., has expressed a new willingness to support "a reasonable expansion of the park"; and 3) Mr. Scott's successor, John Warner, says he favors some expansion of the park.

The legislation is important not just to Virginia, but to the country, for without its enactment, more than 1,700 acres of historically valuable land surrounding the Manassas Park could be permanently ruined by commercial developments. There are local officials, including Chamber of Commerce authorities, eager for "more Gettysburg-type development in and around" the park -- even though there are thousands of unused industrial and commercial acres available elsewhere in that area. Fortunately, other local groups and national organizations support the protection measure.

This year, Rep. Herbert E. Harris (D-Va.), who has been the bill's sponsor since 1976, has attempted to incorproate many suggestions made during months of meetings with private property owners, county supervisors and others. Provisions have been added to protect landowners now living within existing boundaries of the park; and to ensure that land acquired for park expansion would not block consideration of a proposed highway around the park. Yesterday, a House subcommittee approved the bill once again; and today, Sen. Warner is scheduled to meet with Rep. Harris to discuss it. Plenty of time and effort have gone into the measure; it deserves to be passed by the House so that the Senate -- which has never had an opportunity to vote on it -- can act to preserve the Manassas land before it is too late.