The Prince William County Board of Supervisors yesterday proposed a "compromise" in the protracted dispute over expansion of Manassas National Battlefield Park, but proponents of expansion immediately rejected it.

The board has effectively blocked enlargement of the park by enlisting the aid of Virginia's two U.S. senators, who have thwarted Senate approval of a House-passed expansion bill.

Board efforts to present its compromise proposal with official fanfare were undercut by the absence of Interior Department and members of Congress invited to attend a press conference called to announce the plan.

At that press conference in Manassas, Board Chairman Donald White said the county would support expansion of the 3,000-acre park if a tract on its western edge, the Brauner farm, is deleted from the plan and if other privately owned land within and surrounding the park is protected by scenic easements rather than outright purchase by the federal government. The board also said it must be assured that no future expansions will be proposed.

In an allusion to the Civil War battles at Manassas, White said in a prepared statement, " . . . Prince William County officials have taken a stand to preserve their cherished land from the ever invading federal government."

Advocates of expansion contend that the Brauner farm should be incorporated into the park because it was the site of the opening engagement of the Second Battle of Manassas.