Fusion GPS last week demanded that the full transcript be released, sparking a fight with Judiciary Chairman Chuck Grassley (R-Iowa) over who was standing in the way of the document being released. 

The Senate Judiciary Committee interviewed Simpson in August as part of its investigation into Russia's election interference. 

Simpson has emerged as a central figure in the probes into Russian election interference after his firm helped assemble a controversial dossier tying President Trump to Russia.