Legislation to expand Manassas National Battlefield Park, approved by the House for four consecutive years only to be blocked in the Senate, got a big boost yesterday when Sen. Dale Bumpers (D-Ark.) pledged to work for passage of the bill before Congress adjourns this year.

The size of an expanded Civil War park in suburban Northern Virginia remains at issue. A House-passed bill would add 1,715 acres, while a proposal pending in the Senate would add closer to 800 acres. But the major hurdles to passage -- opposition by Virginia's senators and the Prince William (Va.) County Board of Supervisors -- have been removed.

Bumpers, who is chairman of the Senate subcommittee on parks, recreation and renewable resources, presided at a hearing yesterday on a compromise bill sponsored by Sen. John Warner (R-Va.), which also is supported by the state's senior senator, Independent Harry F. Byrd Jr.

Bumpers commended Warner and Byrd "for their desire and willingness to work out a proposal" and assured them that "I will do what I can to ensure favorable action . . . before the 96th Congress adjourns."

The goal of Warner's bill, and the House version, sponsored by Rep. Herbert E. Harris II (D-Va.), is to protect the present park from what Ira Hutchison, deputy director of the National Park Service, called "impending irreparable harm by inappropriate sporadic commercial and residential development."

The existing park covers 4,712 acres, comprising the majority of the main battle areas of the first and second battles of Manassas, which were fought in 1861 and 1862. The first battle was the initial major land battle of the Civil War, and was fought by raw recruits and inexperienced officers. The second battle, lasting three days, was fought by combat-seasoned veterans. It caused the heaviest casualties of the war up to that time, and resulted in a Confederate victory that opened the way for the first offensive campaign in the north by forces of General Robert E. Lee.

Both the Warner and Harris proposals would incorporte three historically significant areas -- Stone Bridge, the Wheeler Farm and the Brawner farm -- into the park.

Most of the approximately 900 additional acres included in the Harris bill are near the entrance to the park, just off Interstate 66 and State Rt. 234.

Warner did not include the extra acreage in his bill primarily because of the cost, about $10 million, and because the land is not historically important. Harris believes the entrance area should be acquired to serve as a buffer between the quiet, rolling parkland and the burgeoning suburban development nearby. The National Park Service has suggested the federal government could obtain scenic easements (barring certain kinds of development) without actually buying the extra land.

Donald L. White, vice chairman of the Prince William board, told Bumpers the county "is opposed to any expression of the park" beyond the 616 acres contained in Warner's original proposal. (Warner yesterday agreed to add about 180 acres to his bill.)

In past years, Byrd and Warner's predecessor, former Sen. William L. Scott (R-Va.), invoked senatorial privileges to prevent the Senate from acting on expansion of the park, largely because of opposition from county officials.