Reductio ad Hitlerum

Reductio ad Hitlerum, also known as playing the Hitler card, is a logical fallacy described by Leo Strauss, a philosopher, in 1951. It happens when someone compares their opponent's view with Hitler or something that he did.
It is a form of ad hominem, distracting from the argument by criticizing the opponent. It works because in the modern world, people think of Nazism as a bad thing.
During the 2016 US Presidential Election, Republican candidate Donald J. Trump has been repeatedly compared to Hitler by commentators and comedians.
An example is as follows:
As person 2 has criticized the opponent as opposed to the argument by comparing them to Hitler without giving any reasons as to why X is not true, they have committed reductio ad Hitlerum.