James Clark, who lost his bid for D.C. mayor last year, won yesterday when a D.C. Superior Court jury acquitted him of charges that he struck a Marion Barry campaign worker over the head with a chair.

Clark, 39, a pharmaceutical technician, was charged with simple assault and possession of a prohibited weapon -- an aluminum chair -- on Aug. 9, 1978, following an incident in which Clark allegedly picked up a chair and struck Philip Ogilvie, a Barry campaign strategist, at a D.C. board of elections hearing.

Ogilvie had challenged the signatures listed on Clark's nominating petition.

Clark testified that he had been kicked twice by Ogilive under the table during the hearing and when he demanded an apology Ogilvie struck out at him. In an attempt to deflect Ogilvie's blow, Clark said he struck him in the jaw.

At almost the same moment, Clark said, someone else in the hearing room threw a chair which struck Ogilvie.

But David J. Gaines, an official reporter for the meeting, said he saw Clark pick up a metal chair and "lower it with force over Ogilvie's head."

Attorney W. Edward Thompson built Clark's defense on the theory that Mayor Marion Barry Jr. had personally ordered his campaign workers to find ways to remove Clark from the coveted first position on the primary election ballot. Clark had won the spot in a draw of lots. Barry was placed second.

Barry, who was subpoenaed to testify, denied that he attempted to have Clark removed from the ballot.