Aided by what one legislator termed "a war room" of supporters, a Virginia General Assembly committee today approved the proposed merger of George Mason University and the International School of Law in Arlington.

The measure cleared the Senate Education and Health Committee by a vote of 12-2. It had been steadily picking up the backing of legislators, many of whom initially were skeptical or openly hostile to the idea of letting the Northern Virginia University open its own law school, but the merger now is given an excellent chance for Senate passage and an even chance for approval by the House.

Chief sponsors of the bill expressed guarded optimism that the 450 students at the unaccredited law school soon will be allowed to affiliate with the university that has 11,000 students but no law school of its own.

The measure's future depends largely on its reception by non-Northern Virginia lawmakers, soem of whom are correctly taking credit for getting the controversial bill this far.

"I will say on behalf of Virginia citizens south of the Occoquan River that we are the ones who passed this bill," bragged Sen. Hunter B. Andrews (D-Hampton), who chaired the Senate committee that approved the measure. Even the two Senate committee members who voted against the bill all but apologized for their opposition, with one senator calling his stand "the toughest vote of the session."

George Mason University has been seeking the assembly's permission since 1972 to open its own law school and its latest bid to merge with the financially-troubled International School of Law (ISL) is opposed by the State Concil of Higher Education.

Dr. Gordon K. Davies, the council's director, testified before a House committee today that the state has to set priorities on its educational spending. He said it would be hard to judtify another law school when Virginia has four such accredited institutions already.

But supporters of the merger proposal, among them J. Wade Gilley Jr., state secretary of education, said acquiring ISL for George Mason would save the university an estimated $7 million- $11 million in start-up costs for a new law school. Under amendments approved by the Senate committee, the law school building and some surrounding property would be given to the state and its programs supervised by the Council of Higher Education. The school itself would not seek any state funding until 1980.

Saying that Washington provides special outlets and demands for lawyers, Gilley told the House committee the state must stop telling Northern Virginia students "to go across the river" to law school in Maryland and the District of Columbia.

Supporters of the bill say they have enough votes on the House committee to pass the measure but want to see it approved in the Senate first before bringing the legislation to the House floor.

Del. W. L. Lammon (D-Marion), chairman of the House committee, made it clear today that the student lobbyists from the law school and other merger supporters are making a favorable impression.

"The people who have done the work on this have worked hard and made a good case," Lammon said. "They have a war room up there (in Northern Virginia) I'm told."