As a member of the Senate Judiciary Committee, Senator Arlen Specter has often justified his voting for or against judicial nominees on grounds that he supports those nominees whose views are in the "mainstream," as distinguished from those whose views are "extremist."Now that he is in line to become chairman of that committee in January, because of seniority, the meaning of these two elusive — and elastic — terms becomes crucial.Senator Specter voted against the confirmation of Judge Robert Bork and for the confirmation of Judge Antonin Scalia to the Supreme Court, even though their voting records on the Circuit...