The Stylistic Argument
Proponents of the JEDP theory claim its truth can be demonstrated by the stylistic differences among the documents.  Yet these differences mostly settle down to the fact that certain parts of the Pentateuch are statistical or enumerative, while other parts are narrative, and most of Deuteronomy consists of exhortation.  There is no reason why the same writer should not use each of these styles depending on the nature of the particular subject matter.  Similar instances of the use of styles at least as different as these could be found in the works of nearly any extensive writer of recent years.