I don't think that one person can just fire off a nuclear weapon without others signing off on it. If that were the case, we would be in for a whole new world of problems. Shooting off a nuke will just start a global war, as we are not the only country with the weapons.
There are enough safeguards in connection with nuclear weapons, and the thought that a President can just push a button to start a nuclear war is a myth. This is all baseless rhetoric used by people to foster fear to get them to come to their side in a political argument. Settle down already.
The decision to launch a nuclear weapon can have devastating consequences, and long term effects. So, there needs to be a series of safeguards to protect our lives and the lives of folks around the world. I'm not sleeping well knowing that someone as erratic and volatile as Trump has the potential to unleash WMD.
The answer is deceptively simple: Yes, he can. No matter what. The president has basically unconstrained authority to use nuclear weapons, a seemingly insane system that flows pretty logically from America’s strategic doctrine on nuclear weapons. The US needs a system to launch weapons fast for deterrence to work properly, which means one person needs to be able to order the use of nukes basically unencumbered. The president is the only possible choice.
This entire system, however, is built around a basic faith in democracy: that the American people will elect a sober, stable leader who understands the seriousness of nuclear weapons.