American isolationism did not exclude defending the Pacific

Would an isolationist United States permit Roosevelt to declare war if Japan attacked Allied interests but not an American base?

Likely yes in the Pacific, because American isolationism focused heavily on avoiding a European war rather than abandoning Pacific interests.

American isolationism before World War II largely opposed entry into another European conflict rather than all overseas military action. The Pacific remained a direct American area of interest, as sanctions and the defense of the Philippines already demonstrated. Roosevelt could therefore gain support for war against Japan even without a direct strike on the continental United States. Germany's later declaration of war made the European side politically easier, but the Pacific case stood on different foundations.