I love and employ logical deduction and induction often, and find that nearly every belief on the political spectrum falls into some sort of fallacy. One of the most common is the ad hominem or reverse ad hominem (appeal to authority), in which people speak about the source of the information giving veracity to the information itself. Of course, this is fallacious.
The problem is, it is becoming more commonly true that the source of information is enough to rebut it entirely. Take this piece from the IMF, in which a “study” shows the more money you take from the rich, the better an economy does. The books are cooked here, and the IMF has a vested interest in making sure that income taxation continues or increases. The entity itself is in the business of profits-via-taxation. One need not take the time to defend against the presumptions in such a worldview, because they have been concocted with a conflict of interest.
The moral of the story? Ad hominem can be appropriate, when the conflict of interest is great enough. It is a fine line, but there you have it…