In medieval times, sometimes they would drive a stake through the heart of a corpse to make sure that it would not rise again and terrorize the villagers. McCain is arrogant, bitter, and ornery enough to fight with Death himself. He may actually win. I doubt anyone put a stake in McCain's heart. I hope he doesn't come back.
That guy was so utterly, completely, and entirely obsessed with lust for power down to his very soul, I wouldn't put anything past him. If anyone can bull-horn their way through the veil, it would be McCain.