Capping several weeks of political struggle, the Loudoun County Board of Supervisors voted Tuesday to hold a March referendum on a $31.5 million bond issue for construction of a new high school in the Ashburn area.

The 7 to 1 vote, with Supervisor Ann B. Kavanagh (D-Dulles) dissenting, came after failure of a motion to raise the construction funds through a state agency, bypassing a Loudoun referendum.

Officials say the chances of the March 12 ballot issue passing are uncertain. In recent weeks, several supervisors and others in the county have questioned the size and cost of the high school project, and whether the need for the school has been established. Referendums on relatively small school bonds have done well in Loudoun, but several years ago a $50 million ballot question lost decisively.

The vote to schedule the referendum was a setback to the Board of Supervisors' Democratic leadership and to the School Board. It followed an attempt by some supervisors to hold off on the construction financing issue and create a citizens' commission to study alternatives to the School Board plans, which would have been an even greater slap at the county leadership.

Also Tuesday, the supervisors voted unanimously to hold a $5 million open space bond referendum in the March 12 special election. If approved, the money will be available to the county government to buy land or purchase easements on property considered to be a sensitive natural area because of its unusual plant or animal species.

The plan to build a new high school in the Ashburn area has been in the works for about a year. Loudoun school officials say they will save about $2 million by converting Broad Run High School to a middle school and building a new high school, instead of improving the current high school and building a new middle school to handle expected enrollment growth.

However, plans to include athletic facilities large enough for regional competitions at the new high school and other aspects of the School Board proposal have led to what several supervisors said this week is the widespread public perception that the plan is too costly at a time of economic stress.

School Board members reduced the estimated cost of the high school this week from $34 million to $32.2 million; $700,000 has already been raised.

In voting this week to require voter approval for $31.5 million in new bonds, the Board of Supervisors reversed a July position. Last summer and this week, much of the debate centered on balancing the county government's responsibility to provide adequate school buildings with the taxpayers' right to decide how their money should be spent.

Supervisor James F. Brownell (R-Blue Ridge), who voted with the 5 to 2 majority in July to bypass a referendum, switched his vote this week.

"Things are considerably different than they were in July," Brownell said Tuesday, noting that the Nov. 6 special election resulted in Republican H. Roger Zurn Jr. handily defeating Democrat Howard P. Smith for the Sterling District seat.

"The voters want a say in these major expenditures," Brownell said, shortly after Zurn made similar comments in his first major test as a board member. Joining Zurn, Brownell and fellow Republican Steve W. Stockman (Broad Run) in opposing bonds without a referendum were Supervisors Betsey Brown (D-Catoctin) and Thomas S. Dodson (D-Mercer).

Voting to endorse the supervisors' July position were Chairman Betty W. Tatum (D-Guilford), Vice Chairman Charles A. Bos (D-Leesburg) and Kavanagh. Based on the July vote, the school system has authorized preparation of an architect's schematic plans for the high school, at a cost of about $155,000.

On Tuesday, Dodson withdrew his motion for a citizens' commission to consider alternatives for the high school plan, conceding that the School Board is such a panel. However, he and some other supervisors suggested that the School Board consider setting up such a panel to help win approval of the March referendum.

Bos agreed with many of his colleagues that a new high school is needed, but he added, "The School Board has a long way to go in terms of refining its package" of construction plans.

Kavanagh said she will support the March referendum and noted the political danger if the board had put the issue to the voters in November 1991, when the entire Board of Supervisors will be up for election. A referendum next fall "would be a tremendous political football that would literally rend this county in half," she said.