A half-truth has been circulating concerning the US presidency for quite some time now. The demi-truth is that the US President can do whatever is necessary. However, the potential for extreme action is strictly conditioned.
The US Presidency is like a police commissioner. His precise functions are ill-defined but he has a clear aim: preserving and protecting the Union of the US symbolized by the Constitution. He has much freedom in acting but little freedom with respect to his place in the US governmental structure. Any action he takes is determined by the level of the threat to the Union.
Extreme action can only be justified on the basis that those whom the action is directed against are in a position to disrupt the functioning of the Union. That is the precedent set by Presidents during major wars. It is also worth noting that Nixon erroneously pleaded Presidential prerogative during the Watergate scandal.
Both camps, those who say the US President can’t take any illegal measures and those who say he has unlimited scope, are wrong. At the moment, however, the possible attacks faced by the US are not existential threats and therefore not enough to justify the current heavy-handedness.