“ Before there may be proceedings to punish appellants for failure to register with the Attorney General, the SACB must first find that the DuBois Clubs is a Communist-front organization and issue an order to that effect. ”

“ It is evident that Congress has provided a way for appellants to raise their constitutional claims. But appellants, denying that they are within the coverage of the Act, wish to litigate these claims in an injunctive proceeding in the District Court. The effect would be that important and difficult constitutional issues would be decided devoid of factual context and before it was clear that appellants were covered by the Act. ”