WHEN YOU COME into a courtroom as a defendant or a civil litigant, you are entitled to two assumptions about the judge. You are entitled to assurance that he is neutral about you as an individual, but also that he has a lively and vigilant regard for you personal rights. Those two assurances together make up the presumption of justice, and a person who cannot deliver it is not qualified to be a judge. That is why James M. Thomson ought not to be appointed to Alexandria's Circuit Court.

Mr. Thomson was in the Virginia House of Delegates for 22 years, rising to the position of majority leader. There he compiled a record on basic questions of rights that offended steadily larger proportions of Alexandria's people until finally, last year, they voted him out. In the 1950s, Mr. Thomson was an enthusiastic recruit to the Byrd organization's long campaign of massive resistance to school desegregation. In those years, he argued that mixing of the races "is contrary to the laws of nature."

His rise to prominence in the General Assembly began with his chairmanship of a committee to investigate the NAACP and other groups that were working in behalf of the school desegregation that the Supreme Court had commanded. His hearings, held behind closed doors, turned into a skillful campaign of harassment. The committee cited one witness for contempt in refusing to answer its questions; when the Supreme Court predictably overturned the conviction, in 1959, Thomson commented: "With the complexion of the court the way it is-protecting subversives and people interested in integration-what other result could you expect?"

Well, you might argue that the 1950s were a long time ago. So they were. Moving on to the 1960s, we recall that Mr. Thomson, as the dominant figure on the board of the Alexandria YMCA, tried to turn it into a last enclave of segregation among the city's social services. By 1963, it was the only one of the 143 UGF agencies in the metropolitan area that was still segregated.

Mr. Thomson was eventually beaten on all of those issues-by the courts, by the UGF, by other citizens. The Alexandria schools are now an example of intelligent desegregation, and the YMCA has had black members for many years. Mr. Thomson has not withdrawn his stated views on the rights of blacks, but he has stayed off the subject in recent years. Instead, he has turned his attention to the rights of women. An adamant opponent of the Equal Rights Amendment, Mr. Thomson was a central figure in the maneuvering to prevent the ratification resolution from ever coming to a vote in the General Assembly. As majority leader, he helped block the ratification drive in early 1977. In response, a large number of outraged women joined the vigorous-and successful-campaign to vote him out of office in that year's election. It was one of the very few elections in this region, in this decade, to turn clearly and exclusively on the issue of civil rights.

Virginia's judges are still appointed by a vote of the legislature. Mr. Thomson's old friends in Richmond are ready to give him a judgeship in Alexandria-partly as a gesture of sympathy, partly to punish the city for having rejected him. The only hope of forestalling this unfortunate appointment lies in the possibility-a thin one, at best-that the Alexandria Bar Association will decline to endorse him when it meets on Jan. 4.

It is kind of decision usually made on the basis of friendships and political debts. But this case is out of the ordinary. The appointment of Mr. Thomson promises to do harm to the bar association that would support him, to the bench that he would join, and to the cause of justice itself in Alexandria.