If Jesus Mythicicism is still regarded as "fringe", then a corresponding rejection of an historical Paul is on the far fringe of the fringe. It is a position advocated by Hermann Detering, a German pastor whose doctoral thesis was on Dutch Radical Criticism, and is bounced around by a very few others.
Now Richard Carrier has waded in on the Pauline question with a firm "Nein!" to Detering's "Paul Mythicism".
The best formal attempt to argue for the non-historicity of Paul is that of Hermann Detering (see The Fabricated Paul). I cannot ascertain his qualifications in the field. But his writings are well-informed. They just trip over logic a lot. His case is not sound. Nor is anyone else’s I've examined. They falter on basic methodology (like ignoring the effect prior probability must have on a conclusion, or conflating possibility with probability) and sometimes even facts (e.g., Detering seems to think self-referencing signatures commonly appear only in forgery; in fact, they are commonly found on real letters—I've seen several examples in papyrological journals).So what's the difference between doubting an Historical Jesus and an historical Paul?
Jesus belongs to several myth-heavy reference classes. He is a worshipped savior deity. He is a legendary culture hero. He is a Rank-Raglan hero. And he is a revelatory archangel (already as early as the earliest writings we have, granting the letters of Paul are such). All of those classes of person already start with a high prior probability of being mythical, because most members of them are mythical (or for culture heroes, about even). And these are beings all of whom are claimed to be historical, yet are usually in fact mythical. Just like Jesus.
Paul does not belong to any such class. Paul thus falls into the class of ordinary persons who wrote letters and had effects on history. In ratio, most of such people claimed to exist, actually existed. By far. The mistake being made then is that people assume the starting prior for anyone claimed to exist is “50/50″ (agnosticism) but we know for a fact that that is not true. Examine thousands of cases, and you will find persons claimed to exist, overwhelmingly actually existed.Along the way Carrier takes a intemperate ballistic swipe at James McGrath ("that renowned fool") before addressing in more irenic fashion a variety of issues raised by Bob Price, author of The Amazing Colossal Apostle, who regards little if any of the Pauline writings as genuine. I'd feel aggrieved on James' behalf if I didn't know he was more than capable of baring his own gnashers.
My own brief and clumsy comments on Detering's The Fabricated Paul were posted here in 2012. Not surprising then that I find Carrier's critique quite convincing.
If you're one of those sad individuals who, like myself, finds these issues utterly fascinating, you're unlikely to be disappointed.
That link again: Richard Carrier. The Historicity of Paul the Apostle.