There is no guarantee that Ubama would debate Newt (or any other candidate) to begin with.
There is no law that says that he has to.
True, but failure to debate would give Newt a TON of ammo and fodder and probably result in a massive public backlash, as debates have become part and parcel of Presidential elections.
While it would violate his Alinsky rules by taking him out of his comfort zone, the court of public opinion pretty much mandates the debates.