In the study of logic, one thing we learn is the logical fallacies. A logical fallacy is a mistaken way of thinking. Logical fallacies have been identified— often in ancient times— so we can learn not to make mistakes like that.
When you hear politicians making a personal attack upon other politicians, this is often an ad hominem attack. This is how it works. When you are discussing something— a proposed political policy— and perhaps you feel you don’t have a strong argument— instead of discussing the issue, you start discussing how evil the guy with the other position is.
Why is that a fallacy? Because bad people can hold a good idea just as good people can have bad ones. So, therefore, if you prove another person is a wife-beating swine, it doesn’t prove that the swine’s tax policy proposal is wrong.
You can talk about the policies or ideas of another person, or you can talk about the person himself. When you respond to a statement about an idea with a condemnation of a person with that idea, you are changing the topic. In a way, you may be admitting that you don’t have any good reason to reject the idea you are discussing, when you change the subject by starting a personal attack.
Another way to do something on the line of an ad hominem attack is to associate the disputed idea with a person who is generally regarded as objectively bad. The most popular bad person to use here is Hitler. The idea is bad, you claim, because Adolf Hitler was in favor of it. Or might have been in favor of it. Or was against it but might have changed his mind.
This is a somewhat indirect ad hominem attack. Instead of directly saying Joe, the guy who made a proposal for a tax reform, is a bad man and so his idea must be bad, you connect the idea to Hitler, and (usually) don’t actually say that Joe is just like Hitler for making the proposal.
The ad hominem attack is a type of logical fallacy which is called a non sequitur. Non sequitur means ‘it does not follow.’ In other words, it’s something that is not the point.
Now, if a human being— perhaps a politician— can be proved to be a swine, a racist, an adulterer, a liar, or corrupt, you can certainly mention such things when you evaluating the man’s character. It’s just that the man’s character does not affect whether his individual policies are good or bad. When talking about the policies— the ideas— the character of an individual who has these ideas is a non sequitur.