It’s true that a President saying “the election is cancelled” has no weight of law, but we can postulate a worst-case scenario where a bonkers President starts dispatching federal troops to bust up polling stations and whatnot. Those sorts of shenangians have happened in plenty of places, though I personally doubt the U.S. could ever get that far.
Reading some of the Pit threads about Bush and Rumsfeld suggests quite a few SDMB members feel otherwise.