Fed Chair Powell Says Concern Is About Series Of Legal Attacks On Fed; This Has Nothing To Do With Verbal Criticism By Elected Officials; Legal Actions By Administration Unprecedented; Worry Attacks Are Battering The Institution; Ongoing Threats Of Additional Action Threatens Fed's Ability To Do Job; Important That People Can Depend On Fed Free Of Political Influence; That's Part Of American's Economic Foundation; Will Leave Fed When I Think It's Appropriate To Do So