Here’s Time’s cover. I certainly hope this primary can come to an end without anyone getting decapitated. But the correct reference is “there can be only one” in the rarely used subjunctive tense so as to emphasize the mythic nature of the dictum.
Meanwhile, it’s worth observing that it’s not totally clear that there really can be only one. There are a lot of good reasons to think an Obama-Clinton unity ticket would be an awkward, inconvenient pairing. But at the same time, Reagan-Bush was an awkward inconvenient pairing and that didn’t stop it from happening. And it’s not as if JFK and LBJ were soul mates or anything. If Obama and his team conclude that Obama-Clinton is the only way to make sure that everyone’s rowing in the same direction then you could easily imagine them deciding that consolidating all the Democratic elites behind the ticket is more important than the fact that a Clinton VP choice would wreak havoc with his message.