The candidacy of former Del. James Thomson for an Alexandria judgeship, considered virtually unstoppable three weeks ago, was angrily attacked yesterday by an informal coalition of blacks, women and local lawyers campaigning to block his nomination.

At a joint press conference, representatives of the National Association for the Advancement of Colored People and the American Civil Liberties Union denounced Thomson's views on civil rghts and lack of courtroom experience.

Victor M. Glasberg, chairman of the Northern Virginia ACLU legal panel, called Thomson's record on civil rights an "abomination... abysmal."

Asked if Thomson's appointment would set the Alexandria bench back 200 years, as suggested recently by an Alexandria lawyer, NAACP chapter president Ulysses Calhoun said, "... farther than that."

Thomson, who has continued his campaign to gather support in spite of mounting opposition among some local attorneys, said yesterday, "I'm not in the least bit upset or discouraged by the antics going on or the way the press is reporting them."

Thomson said his detractors "have their opinions. They're entitled to their side."

Local members of the National Organization of Women and Virginians for the Equal Rights Amendment yesterday echoed the anti-Thomson statements.

"We shrink with horror at the thought of any woman appearing before Thomson as a victim of a crime..." a NOW statement said in part.

The feminist organizations said they will join NAACP members in picketing the Alexandria Bar Association when it meets at City Hall next Thursday to endorse a candidate for the judgeship.

Thomson, 54, a former House majority leader and 22-year legislative veteran, is seeking the seat of chief Circuit Court Judge Franklin P. Backus, who will resign Feb. 1. Thomson was defeated at the polls a year ago after local ERA supporters campaigned vigorously against him.

Alexandria attorney Thomas Moncure yesterday praised Thomson as "highly intelligent, an able legislator and a good lawyer." Moncure added that Thomson, whom he has known for 20 years, "is fully qualified for the bench."

Thomson is opposed for the judgeship by local attorneys Albert H. Grenadier, Terrence A. Sidley and James Wools.

The bar's endorsement traditionally has been considered crucial to appointment by the General Assembly, where Thomson still has strong political backing in the Democratic majority.

Outspoken criticism of Thomson in recent weeks by local groups has led to skepticism about his chances in some quarters of the Alexandria legal community.

"I think he's got a real fight on his hands," said attorney John K. Zwerling. "I think there's a good chance he's not going to make it. Thomson should step out of the race in the interest of the dignity of the judicial system in Virginia."

The ACLU's Glasberg said Thomson's candidacy is a "prime example" of defects in the judicial selection system in Virginia.

"A candidate may be selected notwithstanding that he or she may be quite lacking the... legal skills to sit as a judge, or may be a bigot, or... hostile to safeguarding constitutional rights," said Glasberg.

"A leading contender for the Alexandria judgeship has, to date, never repudiated his vigorous support of racial segregation, which dates back two decades and which he has publicly avowed as late as the 1970s." Glasberg added that it was "astounding" that the process, would permit such a candidate to be considered.