Setting aside legality (regardless of whether a president legally can or can't) just for a moment, there's another layer to the can or can't: the people who would physically carry out the order.
I'm not at all going to argue that not one of them would (some would, especially the culture of ego-ed pilots). However, from processes witnessed, I'd share two hurdles that are not prohibitive, but hurdles still--diminishing the 'can':
1) No one person can carry out the order alone, fulfill a launch. It takes at least two (for the easiest, go-go-now, meant-for-response-to-others'-nukes launch). More people are involved for nukes involving planes (whether air-launched or bombs). So one hurdle.
And that's giving the very doubtful assumption that a president either sends the SIPRNet message himself or picks up the red phone himself to directly contact, skipping over all in the chain of command on the way to the specific lowlies with keys.
2) A regular screening question to weed out the eager or the knee-jerk suck-ups is exactly the question being asked here. "Would you turn the key and flip the switch if [condition]? Would you if [slightly altered condition]? Would you if [again slightly altered]?"
I have seen people whisked away and out of the program for answering 'yes' rather than asking a question in response. (Maybe not something you want in a chain-of-command, but there it is.)
The U.S. isn't incapable of having a president get a nuke off by his order alone, but it's not as easy as him tapping the diet coke button on his desk. It's possible as are many things, but unlikely.
Could happen one day--ever more so as the proportion of nons in the military grows and as jews get closer and confident and salivating and wild for a finish.
